Fix VMwareVCDriver to support multi-datastore

This fixes the issue of VCDriver selecting the first datastore of
Cluster while provisioning instances. User can provide a regex to
match with the desired datastores.

Fixes: bug #1104994

Change-Id: I319b0d8afd03e9fef00ea0c8c799790714685104

Conflicts:

	etc/nova/nova.conf.sample
	nova/virt/vmwareapi/vm_util.py