Fixes Glance Registry V2 client

Add an optional configuration step to glance.db.get_api().

This step is then used in glance.db.registry.api in order to ensure that
the registry client has been configured in time for the registry db
driver's use.

Closes bug: #1302345

Change-Id: Id2c52c423b411ec20bdfa76ee91837c98b1613ef
Co-Authored-By: Mark J. Washenberger <mark.washenberger@markwash.net>