Implement restart-policy handling. Closes #1734

Applications without a restart-policy will have their policies set to `on-failure:10`. Users can completely unset the restart-policy using the `docker-options` plugin, though this will be equivalent to setting it explicitly to `no`.

Restart policies must be explicitly set, and the following are all valid:

- no
- unless-stopped
- always
- on-failure
- on-failure:NUMBER

plugins/ps/commands

@@ -16,6 +16,8 @@ case "$1" in
     ps:restart <app>, Restart app container(s)
     ps:restartall, Restart all deployed app containers
     ps:restore, Start previously running apps e.g. after reboot
+    ps:restart-policy <app>, Shows the restart-policy for an app
+    ps:set-restart-policy <app> <policy>, Sets app restart-policy
 help_content
     }