Make gate-tempest-devstack-vm-neutron non-voting

This has been quite painful recently and I propose it be made non-voting
until it gets fixed.  See the following thread for discussion about
this:

http://lists.openstack.org/pipermail/openstack-dev/2013-July/011734.html

Change-Id: I5d00ad9cc6cb869b60850b139965a1089622e0a2
Reviewed-on: https://review.openstack.org/36685
Reviewed-by: Dan Smith <dms@danplanet.com>
Reviewed-by: Sean Dague <sean@dague.net>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins