Let image upload happen after configuration is applied

In order to succesfully upload an image, it is important that the server side
got a chance to react to possibly updated configuration. The Glance_image class
itself also parses the configuration of the API service for stuff such as region
name. When any of these options are changed through the Glance_api_config, this
modification has to be saved on disk at the time the Glance_image class parses
the configuration file.

This patch utilizes the transitivity of the ~> operator where a change to a
Glance_api_config already notifies the Service['glance-api'].

Change-Id: I21d7c40de79b5e40421f9d645236ee972bd061e5
Merge "Let image upload happen after configuration is applied"