linuxbridge-agent: add missing sysctl rootwrap entry
linuxbridge-agent: add missing sysctl rootwrap entry

Sysctl was missing from the linuxbridge plugin rootwrap
configuration file. This was causing failures in the
linuxbridge agent when networks are created:

Rootwrap error running command: ['sysctl', '-w', 'net.ipv6.conf.eth0/557.disable_ipv6=1']:

NOTE: this bug was hidden by the fact that sysctl was
covered by the iptables-firewall.filters until recently,
when it was removed (see https://review.openstack.org/#/c/436315/).

Change-Id: Id20175df30d4d6039fb42e722d03f39521f6a499
Closes-Bug: #1715194

huntxu/neutron,mahak/neutron,huntxu/neutron,openstack/neutron,mahak/neutron,openstack/neutron,noironetworks/neutron,noironetworks/neutron,eayunstack/neutron,eayunstack/neutron,mahak/neutron,openstack/neutron