Fix KeyError on missing gw_port_host for L3 agent in DVR mode

The order of Mixin imports broke the MRO, which caused some methods
in the L3 hierarchy to be ignored. In particular, _build_routers_list
for DVR was no longer called, which led to the stacktrace observed on
the L3 agent side.

Closes-bug: 1369012

Change-Id: I23cd9813fb9b67e9029d222d5f72733ecec3febb
Merge "Fix KeyError on missing gw_port_host for L3 agent in DVR mode"