heat engine : ensure create thread exits on stack delete

Detect GreenletExit exception when creating resources - this may
happen if the stack gets deleted while the stack is still in
CREATE_IN_PROGRESS state.  If we don't detect this then the stack
continues trying to create resources during/after the stack has been
deleted, with predictably nasty results..

fixes bug 1096150

Change-Id: Ieb6ed38d0000cd6fe3bc1af2eb5d2e5eecc74fb4
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : ensure create thread exits on stack delete"