Permit overriding the undercloud controller ip.

Currently we infer the controller IP address from Neutron, but when
we get HA, the controller address may move around so would need to
be the floating IP. Equally, folk doing unusual sets may need to
have the IP address be something other than what the seed Neutron
handed out.

Change-Id: I97441d009b831fd7356a6c58f4c9edc7532f7d05