Implement consistent hashing of nodes to conductors

Implement the consistent hashing of nodes to conductors.

Implement automatic routing of RPC messages from the API tier to the
conductor tier, by using the consistent hash ring.

Improves _sync_power_states to only check the nodes mapped to that
particular conductor.

Add a stub for a rebalance method which can trigger certain actions by a
conductor when a node is mapped to it by changes in the hash
distribution.

blueprint instance-mapping-by-consistent-hash

Change-Id: Ib74a8ded49e4a85964c46d4a445ff0a2df39862a