Revamp link/unlink commands

Previously we were exporting `REDIS_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 `REDIS_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 `REDIS_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_REDIS_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-redis.git redis
 ## commands
 
 ```
-redis:alias <name> <alias>     Set an alias for the docker link
 redis:clone <name> <new-name>  Create container <new-name> then copy data from <name> into <new-name>
 redis:connect <name>           Connect via redis-cli to a redis service
 redis:create <name>            Create a redis service
@@ -34,6 +33,7 @@ redis:info <name>              Print the connection information
 redis:link <name> <app>        Link the redis service to the app
 redis:list                     List all redis services
 redis:logs <name> [-t]         Print the most recent log(s) for this service
+redis:promote <name> <app>     Promote service <name> as REDIS_URL in <app>
 redis:restart <name>           Graceful shutdown and restart of the redis service container
 redis:start <name>             Start a previously stopped redis service
 redis:stop <name>              Stop a running redis service
@@ -58,8 +58,6 @@ dokku redis:create lolipop
 # get connection information as follows
 dokku redis:info lolipop
 
-# lets assume the ip of our redis service is 172.17.0.1
-
 # a redis service can be linked to a
 # container this will use native docker
 # links via the docker-options plugin
@@ -69,24 +67,42 @@ dokku redis:link lolipop playground
 
 # the above will expose the following environment variables
 #
-#   REDIS_URL=redis://172.17.0.1:6379
-#   REDIS_NAME=/lolipop/DATABASE
-#   REDIS_PORT=tcp://172.17.0.1:6379
-#   REDIS_PORT_6379_TCP=tcp://172.17.0.1:6379
-#   REDIS_PORT_6379_TCP_PROTO=tcp
-#   REDIS_PORT_6379_TCP_PORT=6379
-#   REDIS_PORT_6379_TCP_ADDR=172.17.0.1
+#   DOKKU_REDIS_LOLIPOP_NAME=/lolipop/DATABASE
+#   DOKKU_REDIS_LOLIPOP_PORT=tcp://172.17.0.1:6379
+#   DOKKU_REDIS_LOLIPOP_PORT_6379_TCP=tcp://172.17.0.1:6379
+#   DOKKU_REDIS_LOLIPOP_PORT_6379_TCP_PROTO=tcp
+#   DOKKU_REDIS_LOLIPOP_PORT_6379_TCP_PORT=6379
+#   DOKKU_REDIS_LOLIPOP_PORT_6379_TCP_ADDR=172.17.0.1
+#
+# and the following will be set on the linked application by default
+#
+#   REDIS_URL=redis://dokku-redis-lolipop:6379/0
+#
+# 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 redis:link other_service playground
 
-# you can customize the environment
-# variables through a custom docker link alias
-dokku redis:alias lolipop REDIS_DATABASE
+# since REDIS_URL is already in use, another environment variable will be
+# generated automatically
+#
+#   DOKKU_REDIS_BLUE_URL=redis://dokku-redis-other-service:6379/0
+
+# you can then promote the new service to be the primary one
+# NOTE: this will restart your app
+dokku redis:promote other_service playground
+
+# this will replace REDIS_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:
+#
+#   REDIS_URL=redis://dokku-redis-other-service:6379/0
+#   DOKKU_REDIS_BLUE_URL=redis://dokku-redis-other-service:6379/0
+#   DOKKU_REDIS_SILVER_URL=redis://dokku-redis-lolipop:6379/lolipop
 
 # you can also unlink a redis service
-# NOTE: this will restart your app
+# NOTE: this will restart your app and unset related environment variables
 dokku redis:unlink lolipop playground
 
 # you can tail logs for a particular service