heat engine : make watchrule actions run in stack ThreadGroup

Make watchrule alarm actions run via greenthreads in the stack
ThreadGroup - this allows them to be correctly cancelled if the
stack is deleted whilst an alarm action is in progress.

fixes bug 1097847

Change-Id: I3190a05e3d0abd492961071d2db59a212bd4d373
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : make watchrule actions run in stack ThreadGroup"