Revamp link/unlink commands

Previously we were exporting `DATABASE_URL` via the docker-args* hooks.
This seems to confuse our users (since the env var is not displayed
when calling `dokku config`) and in some cases it also seems that the
env var is not correctly set.
Another problem is if several services are linked to the same app and
if they are exporting `DATABASE_URL` as well. Then we don’t know what
will be set.

To resolve theses issues, this patch changes the way we manage the env
vars. We use standard dokku commands (`config` and `docker-options`) so
config is set on the linked application and can be reviewed by the user
easily.
We also handle the case where `DATABASE_URL` is already set on the
linked application. When it’s the case, we automatically generate
another env var based on the following pattern: DOKKU_<service
name>_<random unused color>_URL. For example, this can give:
DOKKU_POSTGRES_BLACK_URL.

Since naming is now handled automatically, the `alias` command has been
removed. If the user wants to set a different env var on her app, it’s
just a matter of using `dokku config:set` and pasting the wanted value.

IP in DSN has been removed in favor of host name exported by docker in
the container. This is more robust and simpler since the IP can change
but the name will remain the same if the service container restarts for
instance.

With all those changes, a new command has been introduced: `promote`.
The goal of this command is to easily set a service as the primary one
when several are linked to an app. (see README for an example)

README.md

@@ -22,7 +22,6 @@ dokku plugin:install https://github.com/dokku/dokku-postgres.git postgres
 ## commands
 
 ```
-postgres:alias <name> <alias>     Set an alias for the docker link
 postgres:clone <name> <new-name>  Create container <new-name> then copy data from <name> into <new-name>
 postgres:connect <name>           Connect via psql to a postgres service
 postgres:create <name>            Create a postgres service
@@ -34,6 +33,7 @@ postgres:info <name>              Print the connection information
 postgres:link <name> <app>        Link the postgres service to the app
 postgres:list                     List all postgres services
 postgres:logs <name> [-t]         Print the most recent log(s) for this service
+postgres:promote <name> <app>     Promote service <name> as DATABASE_URL in <app>
 postgres:restart <name>           Graceful shutdown and restart of the postgres service container
 postgres:start <name>             Start a previously stopped postgres service
 postgres:stop <name>              Stop a running postgres service
@@ -57,8 +57,6 @@ dokku postgres:create lolipop
 # get connection information as follows
 dokku postgres:info lolipop
 
-# lets assume the ip of our postgres service is 172.17.0.1
-
 # a postgres service can be linked to a
 # container this will use native docker
 # links via the docker-options plugin
@@ -68,24 +66,42 @@ dokku postgres:link lolipop playground
 
 # the above will expose the following environment variables
 #
-#   DATABASE_URL=postgres://postgres:SOME_PASSWORD@172.17.0.1:5432/lolipop
-#   DATABASE_NAME=/lolipop/DATABASE
-#   DATABASE_PORT=tcp://172.17.0.1:5432
-#   DATABASE_PORT_5432_TCP=tcp://172.17.0.1:5432
-#   DATABASE_PORT_5432_TCP_PROTO=tcp
-#   DATABASE_PORT_5432_TCP_PORT=5432
-#   DATABASE_PORT_5432_TCP_ADDR=172.17.0.1
+#   DOKKU_POSTGRES_LOLIPOP_NAME=/lolipop/DATABASE
+#   DOKKU_POSTGRES_LOLIPOP_PORT=tcp://172.17.0.1:5432
+#   DOKKU_POSTGRES_LOLIPOP_PORT_5432_TCP=tcp://172.17.0.1:5432
+#   DOKKU_POSTGRES_LOLIPOP_PORT_5432_TCP_PROTO=tcp
+#   DOKKU_POSTGRES_LOLIPOP_PORT_5432_TCP_PORT=5432
+#   DOKKU_POSTGRES_LOLIPOP_PORT_5432_TCP_ADDR=172.17.0.1
+#
+# and the following will be set on the linked application by default
+#
+#   DATABASE_URL=postgres://postgres:SOME_PASSWORD@dokku-postgres-lolipop:5432/lolipop
+#
+# NOTE: the host exposed here only works internally in docker containers. If
+# you want your container to be reachable from outside, you should use `expose`.
 
-# you can examine the environment variables
-# using our 'playground' app's env command
-dokku run playground env
+# another service can be linked to your app
+dokku postgres:link other_service playground
 
-# you can customize the environment
-# variables through a custom docker link alias
-dokku postgres:alias lolipop POSTGRES_DATABASE
+# since DATABASE_URL is already in use, another environment variable will be
+# generated automatically
+#
+#   DOKKU_POSTGRES_BLUE_URL=postgres://postgres:ANOTHER_PASSWORD@dokku-postgres-other_service:5432/other_service
+
+# you can then promote the new service to be the primary one
+# NOTE: this will restart your app
+dokku postgres:promote other_service playground
+
+# this will replace DATABASE_URL with the url from other_service and generate
+# another environment variable to hold the previous value if necessary.
+# you could end up with the following for example:
+#
+#   DATABASE_URL=postgres://postgres:ANOTHER_PASSWORD@dokku-postgres-other_service:5432/other_service
+#   DOKKU_POSTGRES_BLUE_URL=postgres://postgres:ANOTHER_PASSWORD@dokku-postgres-other_service:5432/other_service
+#   DOKKU_POSTGRES_SILVER_URL=postgres://postgres:SOME_PASSWORD@dokku-postgres-lolipop:5432/lolipop
 
 # you can also unlink a postgres service
-# NOTE: this will restart your app
+# NOTE: this will restart your app and unset related environment variables
 dokku postgres:unlink lolipop playground
 
 # you can tail logs for a particular service