Set firewall_rules only after appending all rules

The firewall_rules attribute was being re-set per each firewall rule.
Instead if should only be set once when all firewall rules are appended
to _firewall_rules.

A minor PEP E128 (continuation line under-lined) is also being fixed
here.

Change-Id: I9db6d3dc0ad11a3194513d80f15151c29aa019cd
Closes-Bug: 1328625
Merge "Set firewall_rules only after appending all rules"