Enable Cinder as a backend for Glance

Enable Cinder as a backend for Glance

Enable Cinder as a backend for Glance by adding 'cinder' to the list of
allowed choices for the GlanceBackend heat parameter.

Update the glance-api docker configuration to allow the feature to work.
This is necessary because the feature uses iSCSI, which requires additional
privileges.

Closes-Bug: #1728409
Depends-On: I850047e32f3608b3ce490e52e2e540695cb1a4ff
Change-Id: I42241747de931103a04aa5ee2ed18fd46197d183
(cherry picked from commit e828e8c7bb2e890b243faa767992226dc270bb6f)