(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
@anodelman @kevpl - Is a blank/empty option a concern? If so, how or where should that be checked?