Added swift_store_region variable for glance-api.

Now that swift is a global cluster, we can have our glance-api point to
a particular region for the swift image store. Then glance-api will
look for images in the local region first.

The api class has been modified to add a variable, swift_store_region,
whose value is set by glance::api::swift_store_region in hiera. If it's
not empty, this will appear in the Havana glance-api.conf template as
swift_store_region.

(Amendment: cleaned it up a bit.)
(2nd amendment: cleaned up unnecessary whitespace in the template.)

Change-Id: If8714a9a980e3d7f9862acce2516424455016b44