Change location of [glance_store] options

I moved the [glance_store] configuration options from the
glance-registry.conf file to the glance-api.conf file. I
also fixed a minor wording consistency with the optional
step to enable verbose logging.

Change-Id: I730dacc91ceb9fc1b689796dc19f2e79e5b91276
Closes-Bug: #1388379
backport: juno
(cherry picked from commit 829bf95b9fadf25394f41377df4a7ebce471bc74)