openstack-status: fix detection of systemd installed services

`systemctl show` always returns success.
Instead grep the output of the installed services list.
The awkwardness of this is detailed in:
https://bugzilla.redhat.com/748512
openstack-status: list "target" service status

This service is used on newer systems rather than
"tgtd" or "targetd", and is part of the python-trslib package.
Determine the appropriate service to use by querying
the services directly rather than the packages installed.
openstack-service: mention recent change in NEWS