Move default_store option in glance-api.conf

[DEFAULT] default_store was deprecated in Juno and moved into the
[glance_store] section. Yet it remains in the old place in the sample
glance-api.conf. Additionally, some comments still refer to the
known_stores option, which is now simply stores.

DocImpact

Change-Id: I7215e35534fd9a77730d39b96f9ba3bf6c3ea065
Merge "Move default_store option in glance-api.conf"