Implement basic plugin for VM management

This commit implements basic VM management plugin for Climate.
As decided we use nova shelved instances to support our reservation
model. The only one thing we do for "on_start" lease action -
'unshelve' instance. As decided we support configurable opts for
"on_end" lease action, by default it set to snapshot and delete VM.

Implements bp:basic-vm-plugin

Change-Id: Ia34e16c636d1fa8d200873334f55b9868866f97f
Merge "Implement basic plugin for VM management"