For #313: Do not specify both $APP and $IMAGE for base commands.

As discussed in #313, dokku base commands such as build, release, and
deploy require specifying both `$APP` and `$IMAGE` as arguments where
`IMAGE="app/$APP"`. Thus, the `$IMAGE` parameter is currently redudant.

This patch removes the need to specify `$IMAGE` for these base commands
and is backwards compatible. If, in the future, the base commands expect
and `$IMAGE` different than `app/$APP`, then the `$IMAGE` parameter will
be re-added for those specific commands.

plugins/config/commands

@@ -49,13 +49,13 @@ config_styled_hash () {
 }
 
 config_restart_app() {
-  APP="$1"; IMAGE="app/$APP"
+  APP="$1";
 
   echo "-----> Releasing $APP ..."
-  dokku release $APP $IMAGE
+  dokku release $APP
   echo "-----> Release complete!"
   echo "-----> Deploying $APP ..."
-  dokku deploy $APP $IMAGE
+  dokku deploy $APP
   echo "-----> Deploy complete!"
 }