Fix loopback service status

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

.gitreview

+[gerrit]
+host=gerrit.mcp.mirantis.net
+port=29418
+project=salt-formulas/linux
+

linux/files/setup-loopback-device.systemd

 [Service]
 {# The command is prefixed with '-' to consider it a success if the loopback device is already setup #}
 ExecStart=-/sbin/losetup {{ device_name }} {{ file }}
+{# In order to Salt can recognize oneshot service as running, we need it to remain active after it exited #}
+RemainAfterExit=true
 Type=oneshot
 
 [Install]