Some distros no longer ship brctl, iproute2 should be used
in its place. The linuxbridge agent plugin script was still
using it, as was worlddump, which generates this warning on
a failure:
Running devstack worlddump.py
/bin/sh: 1: brctl: not found
Conditionalizing worlddump based on whether brctl is installed
to make this go away.
Change-Id: Iafbf4038bab08c261d45d117b12d4629ba32d65e
da18895162