Add method which returns cluster public VIP by id

When we want to connect to OpenStack services API which
are running on HA cloud, we need to have ability to get public
virtual IP address of deployed environment. So adding an
additional method to the FuelWebClient class for that.

Related-bug: #1369412
Change-Id: I98c8c323dd81f0b361b73050df29083e0d912211
Merge "Add method which returns cluster public VIP by id"