VMware: bug fix for host operations when using VMwareVCDriver

Currently the driver does not have a mapping between a Nova 'host'
and a VMware 'cluster'. A host operation would update the state of
one of the hosts in one of the configured clusters.

The solution is for the driver to raise a NotImplemented exception.

Change-Id: Idd78020374a6fe2d34bf8d860c30e2ed8c719903
Closes-bug: #1274129
Merge "VMware: bug fix for host operations when using VMwareVCDriver"