Fix random fails of test_clean_volumes_after_reset

Wait for deployment and reset task to finish to prevent situation
when not all task has been completed.
Also more reliable way of simulating disk change has been used.

Change-Id: I18e92081994f2f0204710edebbf8aeaaec8b5bce
Closes-Bug: #1387066
Merge "Fix random fails of test_clean_volumes_after_reset"