Separate Overcloud into plan and stack

This patch separates Overcloud out into OvercloudPlan in api/tuskar.py
and OvercloudStack in api/heat.py.  This is in preparation for the
split that will be needed since tuskar is being re-defined as a
planning service.

Change-Id: I9ec9a0be1fded8b6918480858ffbb9a4887a3f70