VMware: get_all_cluster_refs_by_name default to {}

Currently get_all_cluster_refs_by_name returns None when no cluster
is found.
driver.py is using the keys() method without checking whether the
object returned is None. This can potentially end up throwing an
AttributeError.

This patch fixes the issue by returning an empty dictionary when no
cluster is found.

Change-Id: I3293c916d79aaa4de8913e8ab9c609c9c0bb023e
Closes-Bug: #1372672
Merge "VMware: get_all_cluster_refs_by_name default to {}"