Add neutron-linuxbridge-cleanup util

Add neutron-linuxbridge-cleanup util

Removal of empty bridges have been disabled [1] to fix a race condition
between Nova and Neutron where a bridge would be removed if
the only instance using it is rebooted. This means empty bridges
will pile up over time.

This script can be used to periodically remove empty bridges by running it
on compute nodes.

Note: Usage of this script can still trigger the original race condition.
It should be used when you don't expect anyone do be doing operations
on their instances.

[1] Commit 8dd8a7d93564168b98fa2350eedf56acede42b0f

DocImpact: Add neutron-linuxbridge-cleanup util
Related-bug: #1328546
Closes-bug: #1497027
Co-Authored-By: Cedric Brandily <zzelle@gmail.com>
Change-Id: Ieb2796381579ad295abf361ce483d979a53d2bd6