This change updates the way we write cinder.conf when cinder backups
are enabled. If cinder_service_backup_swift_auth is set to 'per_user',
we do not set any url/user/key/tenant options in the config. However,
if cinder_service_backup_swift_auth is set to 'single_user' then all
auth-related options will be written. Some additional variables were
added to allow 'single_user' to work properly.
Note that backup_swift_url is actually the swift storage URL if using
'per_user', or your auth URL if using 'single_user'.
Partial-bug: #1399386
Change-Id: Ia9d2e140d629b1f7b4c668d2f6041f3c440ab826
179f54ce60