Implement ClusterSpec Propagation in TF Master

ClusterSpec propagation is a capability upgrade for TensorFlow that should make
it much easier to (1) build distributed TensorFlow clusters, and (2) handle
node failures. The ClusterSpec propagation capability allows TensorFlow workers
to be booted independently of each other, and with no knowledge about others.
The client can then construct a ClusterDef (ClusterSpec), and then send it
to the TF master at session creation. The master in turn then propagates the
ClusterDef along to all of the workers.
Change: 155159972