NVP LBaaS: check for association before deleting health monitor

    Bug #1243129 suggests to check for associations before deleting
health monitor in DB logic, so NVP advanced LBaaS should also Change
its operations when deleting a healthmonitor.

Change-Id: I57677cc4b65a13df9f72ec689f34771c7af6a57e
Closes-Bug: #1254639
Merge "NVP LBaaS: check for association before deleting health monitor"