changes to support a wider range of vCenter deployments

Added logic to support more use cases when connecting via vCenter, while still allowing for standalone ESXi cases as well.

* Added :datacenter and :host options to the :vm_options hash of a definition file, and related code

* Because ESX hosts are now being specified as targets, the previous key named "host" in the YAML file that referred to the vSphere connection has been renamed to "vsphere_server" (this also happens to match what [fog](http://github.com/fog/fog) does)

* Minor related cleanup