Add prepare, clean_up, take_over methods to deploy

Add three new methods to the deploy interface,
which will be used by the ConductorManager to trigger
(re)building of the deploy environment and updating of
external mappings/dependencies when rebalancing the hash ring.

Also refactors the PXE driver to utilize these new methods.

blueprint instance-mapping-by-consistent-hash

Change-Id: I1bebb895cbe29a7059a9873ef90ee039790c1031
Merge "Add prepare, clean_up, take_over methods to deploy"