add support for host driver cleanup during shutdown

Provides a place for any drivers to potentially
setup graceful-shutdown code.

VMware driver's proper driver-lifecycle code included.
This is critical in environments where the the vmware
driver is setup and torn down at high frequency.

Prevents run-away vSphere by closing stateless
HTTP management sessions gracefully.

Change-Id: I67a91613643540243ab1210b333ed8e121f05802
related to bug: 1262288
Closes-bug: 1292583
Merge "add support for host driver cleanup during shutdown"