Call server volume detach only once

Changes:
 - Removed duplicate delete calls to prevent race condition.
 - Removed duplicate mocked calls from unit-test.

Closes-Bug: #1311533
Closes-Bug: #1298350

Conflicts:
	heat/engine/resources/volume.py
	heat/tests/test_volume.py

Change-Id: I5f16c528652f12440160f03b92f41b76d1c9100c
(cherry picked from commit d1ffbd4bfde0cd6b7a82a48b9d4f59cc8b310bd8)