Currently when Salt starts setup-loopback-device service
systemd executes losetup command and even if it worked
successfully, systemd marks service as inactive(dead).
And Salt considers service as dead. In order to fix
this behavior, we need to add RemainAfterExit=true to
loopback systemd unit template.

Other changes:
.gitreview added

Related Prod: https://mirantis.jira.com/browse/PROD-11760

Change-Id: I6d0c3baf1c22d0a626ff1fd6aaab802c08fb029e