Improve support for EL7 and other related fixes

* Support RHEL7 by removing firewalld before installing iptables-services
* Autorequire Package[iptables-services] for Firewall and Firewallchain types
* Ensure /etc/sysconfig/iptables exists before starting Service[iptables]
Merge pull request #393 from hunner/develop

Improve support for EL7 and other related fixes