Move glance store options to [glance_store] section

Juno moves the store options under a new section called [glance_store],
this change updates glance-api.conf to reflect that.

Change-Id: Ie4b2484fd48f81add75781616945e1d13b765eaa
Closes-Bug: #1399423
Merge "Move glance store options to [glance_store] section"