Heat SwiftResources : only count containers created by Tempest

Some containers in the demo tenant could be there before the Tempest
run so we should exclude them when we count containers created by
the stack.

Change-Id: I20ae92e8cc2b61197774963d049fd8231dafdb56
Closes-Bug: 1360832
Merge "Heat SwiftResources : only count containers created by Tempest"