Add check to see if the firewall is enabled

[vpetersson]

One of the security assessments we want to do is to check if the firewall on the device is enabled. There are a few ways to do this, but I would recommend using python-iptables to asses the firewall.

In the first iteration, i just want this function to return something like:

{'firewall_enabled': Bool}

Later on, we want to expand this functionality with more detailed analysis.

[vpetersson]

@slilo Tests fail https://circleci.com/gh/WoTTsecurity/agent/15