DVR isn't supported by the Linux Bridge agent, but the
mechanism driver is enabled by default, so Neutron attempts
port-bindings for it, generating ERRORS in the neutron-server
log in the check and gate jobs. Just remove it in the DVR case.
Change-Id: Ic50e12e5fecf366a182c141b5c99649e653254cb
Closes-bug: #1716782