Mock wait_for_volume_removal in test_brick_connector

Mock wait_for_volume_removal in test_brick_connector

Systems where /dev/disk/by-path did exist were failing unit
test for test_connect_volume, test_connect_volume_with_alternative_targets
and test_connect_volume_with_alternative_targets_primary_error in
cinder.tests.test_brick_connector .  The problem was that the test
cases were mocking out os.path to return exists.  This resulted in
the new code from commit 43029941219505bc3861dd285e3393e08c76b433
never being able to exit the wait_for_volume_removal loop.

To avoid this problem I am mocking out wait_for_volume_removal for these
test cases.  wait_for_volume_removal is tested elsewhere and not relevant
for these test cases.

Change-Id: Ib7f75c852f9c79f261e319be0c1e3fd21acab8f1
Closes-bug: 1440153