openstack-service support for @instance services

[apevec]

(copied from https://bugzilla.redhat.com/show_bug.cgi?id=1394689 )

$ sudo openstack-service restart
Failed to restart openstack-zaqar@.service: Unit name openstack-zaqar@.service is missing the instance name.
See system logs and 'systemctl status openstack-zaqar@.service' for details.

$ systemctl status openstack-zaqar@.service
Failed to get properties: Unit name openstack-zaqar@.service is missing the instance name.

I have 3 services running that belog to zaqar:
openstack-zaqar.service
openstack-zaqar@1.service
system-openstack\x2dzaqar.slice