skip test_stack_update_add_remove because of race

test_stack_update_add_remove races about 25% of the time in the
gate because the update is eventually consistent, and doesn't
happen before the list returns. It's unclear if this is
expected or unexpected behavior for heat, but 25% failure rate
is far too high.

Change-Id: I0cfd9c10c1fd5b674c1a2211f98a6aba042227da
Related-Bug: #1308682
Merge "skip test_stack_update_add_remove because of race"