Workaround to fix Heat stack deletion bug in Shade

Workaround to fix Heat stack deletion bug in Shade

There is a bug in Shade, in the function "event_utils.poll_for_events".
While Shade is waiting for the Heat stack to be deleted, retrieves
periodically the status of this Heat stack. When the stack is deleted,
the function "get_stack" [1] returns None; then L90 tries to access to
a key of this None object.

[1]https://github.com/openstack-infra/shade/blob/e5bcb705c5b32334cdc1dd9807126298c87b0622/shade/_heat/event_utils.py#L89-L90
Shade function

JIRA: YARDSTICK-1015

Change-Id: I56ea853be41bfd7f63e094d7a3696d97ba078ba4
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>