This changeset allows to change default store to cinder
and define additional key/values for glance-api.conf file.
Some cinder backends could require additional changes
in service and additional changes in the OS.
Same functionality implemented in cinder charm via storage-backend interface.
Change-Id: Ia9c78df7e9a1ccb0d9ca2121dd01e5832186bf19
2b3eea98eb