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

This state allows to configure loopback device(s). This isn't meant to
be used in production but offers a cheap way to test Cinder with LVM
volumes.