Add delete operations for the ODL MechanismDriver

This commit adds delete operations (networks, subnets and ports) for the ODL MechanismDriver.
It also modifies sync_single_resource to reduce db operations.

Change-Id: I03ca04c83ac2ef9c879fbd87e74bae495daea16d
Closes-Bug: #1324450
Partial-Bug: #1325184
Merge "Add delete operations for the ODL MechanismDriver"