Merge branch 'centos' into 'master'

Merge branch 'centos' into 'master'

Support for centos 6 and 7

I've retested this on rhel 6.5, rhel 7.0, centos 6.5 and centos 7.0.

Centos was able to share a majority of the config with rhel, so I moved what was common into a yum_systems.yml file.  I also made a switch to only storing sshd_configs for each distro's major version - it was getting difficult to maintain a sshd_config for each minor version of a distro.  Plus, I noticed rhel 6.4 and rhel 6.5 had the same sshd_config anyway.

There are also quite a few file renames from rhel_* to redhat_* so that I can dynamically import based on ansible_distribution. These files are otherwise unchanged.