Implement cancelling of stack update

This patch adds a 'cancel' boolean parameter as possible request
parameter and implements triggering of stack update rollback upon
receiving cancel as True.

Change-Id: Ic929d42946cf28eeb2a7caea8bf908f492693c09
Implements: blueprint cancel-update-stack
Implement CFN CancelUpdateStack action

Change-Id: I7a4d9de34448f6f3ad964e36347a80a2f2140251
Implements: blueprint cancel-update-stack