Fix FWaaS plugin to allow one firewall per tenant

The reference implementation of the FWaaS iptables
agent/driver supports only one firewall per tenant
in Havana release. However, the FWaaS plugin will
let you create more than one firewall. This is
being fixed in this patch to not allow creating
the second firewall if a firewall already exists
for the tenant.

Change-Id: I8f1cad9791723ba919b5774a63982c204686ddfe
Closes-Bug: #1228442
Require oslo.config 1.2.0 final

Now that 1.2.0 has been released, require it rather than one of the beta
release tarballs.

Closes-Bug: #1182861
Change-Id: I75ffc4a572d14488d19491a38742d6920951c171
Merge "Require oslo.config 1.2.0 final"
Merge "Fix FWaaS plugin to allow one firewall per tenant"