(QENG-1548) vcloud hypervisor improvements

This PR has two changed to the 'vcloud' hypervisor:

* 'resourcepool' is not a required parameter, as non-DRS-enabled clusters do not have resource pools
* if 'name' is a specified host parameter, a randomized hostname is not generated
We don't tend to handle the 'this space left intentionally blank' error case - so I don't mind this breaking elsewhere.  If you don't want it set, leave it out of the config.