Teach DHCP Agent about DVR router interfaces

When DVR is enabled and enable_isolated_metadata=True,
the DHCP agent should only inject a metadata host route
when there is no port with the gateway IP address configured
on the subnet.  Add a check for DEVICE_OWNER_DVR_INTERFACE
when we look at each port's device_owner field, otherwise
it will always add this route to the opts file when DVR
is enabled.

Change-Id: I3ff3bb85105b8215b36535983016d8c0ff3d8cb7
Closes-bug: #1377307
(cherry picked from commit 29250949012e9c0a60b0ddb56ddbf18d7b68106b)