Fix vmq_server termination error

[dergraf]

When terminating vmq_server it will disable the metadata plugin. This doesn't work as a not existing vmq_plugin_mgr:disable_plugin/2 is called. This wasn't noticed as the exception was catched when the application controller was stopping the vmq_server application.

Changing the disable_plugin to the proper arity revealed another issue. While vmq_server is now able to disable the metadata plugin, the metadata plugin (itself an OTP application) has to be stopped by the application controller. However the application controller blocks until the vmq_server was properly terminated. Resulting in a deadlock when shutting down the vmq_server application.

The workaround that works is to terminate the metadata plugin in it's own process allowing the parent application:stop call to finish.

[larshesel]

Was fixed in #829