Move OpenStack server settings to OpenEdXAppConfiguration.

Created by: smarnach

This PR moves the OpenStack server settings from OpenEdXInstance to OpenEdXAppConfiguration. The latter is an abstract model that both OpenEdXInstance and OpenEdXAppServer derive from, and the code to create a new app server automatically copies over all settings from the instance.

This change makes sure that each appserver keeps a record of the OpenStack settings it actually used.

The migration has been handcrafted to first introduce the fields in OpenEdXAppServer with empty default values, and then immediately modify the fields with the actual default from the settings. This makes sure that existing app servers get empty default values instead of using the current value from the settings, which might not match the settings that were in effect at the time the appserver was created. It is better to have no value at all for historical appservers than to have wrong values.

For testing, run the migration and make sure old app servers don't get any values set for the new fields. Than create a new app server for any instance and make sure the settings get copied over correctly.