CI: Test SSH connectivity to deployed instances

Add testing of the dataplane to the overcloud jobs. To support both
baremetal provisioning and VMs with VXLAN tenant networks, we use the
provision-net as our external network to which floating IPs are
attached.

We depend on a backport of parts of this patch to allow testing of SSH
connectivity after upgrades.

Depends-On: https://review.opendev.org/708915/
Depends-On: https://review.opendev.org/709145/

Change-Id: I6316e8959cff987e4e97280889e1038a9537ed32