DHCP6 Tests fail to remove DVR ports

The method _clean_network() only removes centralized
router interface ports since it uses only 'network:router_interface'
as a valid router interface port.

Change-Id: Ic647d27c5c4e3bcb8ec0349e31684204e3d0c569
Closes-bug: #1403983
Merge "DHCP6 Tests fail to remove DVR ports"