Validation for distro and distro_version

[andrewschoen]

Provide some simple validation for distro and distro_version. For example, if 'centos' is a known and accepted distro and that '7' is a valid distro_version for centos.

[alfredodeza]

I think this is something we need to do in shaman, no need to do it in chacra. Same arguments as issue #187