[Vsphere] added list/get compute resources functions

This patch is to add list/get compute resources of vsphere functions to fog. 

Usage:
to list all compute resources: compute.list_compute_resources
to just list effective compute resources(poweron + connected + not in maintenance state) : compute.list_compute_resources({:effective => true})
to get compute resources by name: compute.get_compute_resource('xxx.xxx.xxx.xxx')

Notice:
Vsphere abstracts clusters and hosts that don't belong to any cluster evenly as compute resource, so the return result may refer to either cluster or single host. If you want to get only hosts or clusters, you can use attribute 'numHosts' to determine the type.