Refactor driver interface to encapsulate the orchestration
strategy. This first patch only refactors the main driver
operations. A follow-on will handle the state synchronization
and removing the poller from the conductor.
1. Make driver interface abstract
2. Move external cluster operations into driver interface
3. Make Heat-based driver abstract and update based on
driver interface changes
4. Move Heat driver code into its own module
5. Update existing Heat drivers based on interface changes
Change-Id: Icfa72e27dc496862d950ac608885567c911f47f2
Partial-Blueprint: bp-driver-consolodation
7890725c52