VMware: Multiple cluster support using single compute service

To allow a single VC driver to model multiple clusters in vCenter
as multiple nova-compute nodes. The VC driver will be configured
to represent a set of clusters as compute nodes.
For example to specify two clusters named cluster-A and cluster-B,
the nova.conf should have:
    cluster_name = cluster-A
    cluster_name = cluster-B

DocImpact
Change-Id: Ia5464948cc30b9b744450f9c301c4f3afaff717b
Blueprint: multiple-clusters-managed-by-one-service
Merge "VMware: Multiple cluster support using single compute service"