VMware: remove ESX support for getting resource pool

The ESX driver is no longer supported. We should only return the
resource pool for the cluster.

In order to test this the patch had to update the test_vmops to
ensure that a cluster was used.

The patch also drops the method get_mo_id_from_instance which is
no longer used.

Change-Id: I9d99a5be58ef5a2a7d41dfd488ac24f38d2c79d3
Merge "VMware: remove ESX support for getting resource pool"