Align glance-api.conf rbd option defaults with config

DocImpact: The example glance-api.conf set different defaults than
what was hardcoded in the code. Change code defaults to those
in the config.

Preparation for generating glance-api.conf with oslo.config generator.

Change-Id: I424e6c5984dbdf5082852c40d97ab1e1a04926ea
Merge "Align glance-api.conf rbd option defaults with config"