Fix brctl calls

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
2 files changed