VMware: map one nova-compute to one VC cluster

Back Story : Back in May 16th, 2014 at the Atlanta Summit, there was an action item:
"Fix driver to map one nova-compute to one Cluster (Mandate from Nova Core)"

Current Status: The review [1] in Nova missed the cut-off so we filed another review just for a deprecation warning for now[2].

Details:
- Operators who are already using the older model will be affected and we need a migration path for them.
- Related issue about "VMware: running a redundant nova compute deletes running instances" has a long email thread at [3] and Matthew has some thoughts about how to go about doing this in [4].

So we thought it would be a good idea to try to file a blueprint to cover this work and file a spec for Liberty.

[0] - https://etherpad.openstack.org/p/juno-nova-vmware-driver-roadmap
[1] - https://review.openstack.org/#/c/103916/
[2] - https://review.openstack.org/#/c/163426/
[3] - http://markmail.org/message/es7rzjdmddkrz3tj
[4] - http://openstack.markmail.org/thread/an7rk7alycfhcf22