Description of problem:
When deploying Ceph on Xenial (which uses systemd), the /api/mon/configure endpoint hangs forever. The reason ceph-ansible hangs forever is that it is waiting on the "ceph-mon | collect admin and bootstrap keys" task.

The problem is that ceph-create-keys waits for keys to appear, and the keys do not appear because Ansible has not properly started the ceph-mon service with systemd.

Running `systemctl start ceph-mon@<hostname>` creates the keys and allows Ansible to proceed with the rest of the playbook to configure the monitor.

Version-Release number of selected component (if applicable):
ceph-ansible-1.0.5-5.el7scon.noarch
ceph-installer-1.0.6-1.el7scon.noarch

How reproducible:
always

Steps to Reproduce:
1. Prepare Xenial machines to host Ceph
2. POST to the installer /api/mon/install , /api/osd/install endpoints
3. Verify the package installations succeeded on the cluster nodes.
4. POST to the /api/mon/configure endpoint
5. Check /api/tasks

Actual results:
The /api/mon/configure task hangs. (Running `systemctl start ceph-mon@<hostname>` on the monitor node(s) allows it to continue.)

Expected results:
The /api/mon/configure task succeeds without intervention.

Additional info:
I found this problem in the monitor config step, and we need ceph-ansible to use systemd on Xenial for all services: ceph-mon and ceph-osd.