Improve support for EL7 and other related fixes

* Support RHEL7 by removing firewalld before installing iptables-services
* Ensure all Package[iptables-services] is installed before applying firewall resources
* Ensure /etc/sysconfig/iptables exists before starting Service[iptables]

This is a combination of other pull requests

* #375
* #367
* #365

The changes in #391 would likely be a good addition to this.

I'm using this code in production for CentOS 6.5 and CentOS 7.0.1406.
Instead of doing a collector here, could you update the type's autorequire? https://github.com/puppetlabs/puppetlabs-firewall/blob/master/lib/puppet/type/firewall.rb#L940