cloud-infra: Pre-provisioned slaves and wait before using slave

cloud-infra: Pre-provisioned slaves and wait before using slave

Jenkins OpenStack instances doesn't really help in our case since the
job needs a slave where it can trigger creation of a slave from
OpenStack which will break the downstream triggering of job on same
slave. We instead fallback to pre-provisioned slaves.

Another issue is that when the instance first gets attached to Jenkins
as slave, it takes some time before it becomes fully functional so
we need to wait.

Change-Id: Ic77b81d2bbdd107cd1a506fc1e9f7ec6be6b3c1e