Revamp link/unlink commands

Previously we were exporting `MONGO_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 `MONGO_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 `MONGO_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_MONGO_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-mongo.git mongo
 ## commands
 
 ```
-mongo:alias <name> <alias>     Set an alias for the docker link
 mongo:clone <name> <new-name>  Create container <new-name> then copy data from <name> into <new-name>
 mongo:connect <name>           Connect via telnet to a mongo service
 mongo:create <name>            Create a mongo service
@@ -34,6 +33,7 @@ mongo:info <name>              Print the connection information
 mongo:link <name> <app>        Link the mongo service to the app
 mongo:list                     List all mongo services
 mongo:logs <name> [-t]         Print the most recent log(s) for this service
+mongo:promote <name> <app>     Promote service <name> as MONGO_URL in <app>
 mongo:restart <name>           Graceful shutdown and restart of the mongo service container
 mongo:start <name>             Start a previously stopped mongo service
 mongo:stop <name>              Stop a running mongo service
@@ -57,8 +57,6 @@ dokku mongo:create lolipop
 # get connection information as follows
 dokku mongo:info lolipop
 
-# lets assume the ip of our mongo service is 172.17.0.1
-
 # a mongo service can be linked to a
 # container this will use native docker
 # links via the docker-options plugin
@@ -68,24 +66,42 @@ dokku mongo:link lolipop playground
 
 # the above will expose the following environment variables
 #
-#   MONGO_URL=mongo://l:PASSWORD@172.17.0.1:27017/lolipop
-#   MONGO_NAME=/lolipop/DATABASE
-#   MONGO_PORT=tcp://172.17.0.1:27017
-#   MONGO_PORT_27017_TCP=tcp://172.17.0.1:27017
-#   MONGO_PORT_27017_TCP_PROTO=tcp
-#   MONGO_PORT_27017_TCP_PORT=27017
-#   MONGO_PORT_27017_TCP_ADDR=172.17.0.1
+#   DOKKU_MONGO_LOLIPOP_NAME=/lolipop/DATABASE
+#   DOKKU_MONGO_LOLIPOP_PORT=tcp://172.17.0.1:27017
+#   DOKKU_MONGO_LOLIPOP_PORT_27017_TCP=tcp://172.17.0.1:27017
+#   DOKKU_MONGO_LOLIPOP_PORT_27017_TCP_PROTO=tcp
+#   DOKKU_MONGO_LOLIPOP_PORT_27017_TCP_PORT=27017
+#   DOKKU_MONGO_LOLIPOP_PORT_27017_TCP_ADDR=172.17.0.1
+#
+# and the following will be set on the linked application by default
+#
+#   MONGO_URL=mongodb://lolipop:SOME_PASSWORD@dokku-mongo-lolipop:27017/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 mongo:link other_service playground
 
-# you can customize the environment
-# variables through a custom docker link alias
-dokku mongo:alias lolipop MONGO_DATABASE
+# since DATABASE_URL is already in use, another environment variable will be
+# generated automatically
+#
+#   DOKKU_MONGO_BLUE_URL=mongodb://other_service:ANOTHER_PASSWORD@dokku-mongo-other-service:27017/other_service
+
+# you can then promote the new service to be the primary one
+# NOTE: this will restart your app
+dokku mongo:promote other_service playground
+
+# this will replace MONGO_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:
+#
+#   MONGO_URL=mongodb://other_service:ANOTHER_PASSWORD@dokku-mongo-other-service:27017/other_service
+#   DOKKU_MONGO_BLUE_URL=mongodb://other_service:ANOTHER_PASSWORD@dokku-mongo-other-service:27017/other_service
+#   DOKKU_MONGO_SILVER_URL=mongodb://lolipop:SOME_PASSWORD@dokku-mongo-lolipop:27017/lolipop
 
 # you can also unlink a mongo service
-# NOTE: this will restart your app
+# NOTE: this will restart your app and unset related environment variables
 dokku mongo:unlink lolipop playground
 
 # you can tail logs for a particular service