Add a constant for router interface device owners

Add a constant for router interface device owners

In the absense of a port object that includes a check for whether a
given port is implementing a router interface, this change adds the
ROUTER_INTERFACE_OWNERS tuple containing the relevant DEVICE_OWNER_*
constants.

This change was suggested by https://review.openstack.org/#/c/129865/

Change-Id: I1c45c29617d692cc05b44f7f2c4ec1e5252be303