Fix Storage Backend Wording

The previous wording was confusing, the config setting does not
configure the *only* glance backend but the *default* glance
backend. You can still overwrite it per image, and searching
for glance images involves all configured backends.
Merge pull request #132 from dirkmueller/default_store

+1
Merge pull request #134 from jtwill98/pull-req-release-roxy-master-17620546f850dd64dddb20c9c812824cc5589804

Merge of Latest Pebble commits into Roxy [7/13]