Use zuul firewall rules in gate

Till now we've been flusing iptables in the gates to allow cross node
communication in the multi node ceph jobs. This raised security
concerns, in particular it exposed memcached to the external net.

This patch uses the infra provided role 'multi-node-firewall' in order
to correctly configure iptables. Thanks to Jeremy Stanley and Jeffrey
for help with this.

Closes-Bug: #1749326
Change-Id: Iafaf1cf1d9b0227b0f869969d0bd52fbde3791a0