(#171) Added ensure parameter to firewall class

This change adds the ensure parameter to the firewall class so that
the appropriate iptables service for the operating system can be
stopped. The reason for this change is to extend the class so that
it is not just limited to running the service.

The change to the manifests still made all the rspec tests pass,
so it should not introduce any regressions. More rspec tests were
also added to exercise the new code paths introduced.

This pull request fixes issue #171.