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
(cherry picked from commit c1ed203ccb816ac0a3a0e015d2790ed3aee04564)
Merge "Add delete operations for the ODL MechanismDriver" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse