Avoid dhcp agent race condition on subnet and network delete

Avoid dhcp agent race condition on subnet and network delete

This patch ensures that, when the dhcp agent queries the server
to retrieve and delete its DHCP port, it does so by selecting
for update. This has been done by introducing a new method
that combines the get and delete in one shot.

Closes-Bug:1197627

Change-Id: I1e8a87d7dc1a1cb9309aeefd41619e20f49f95a6