Implement checks for CentOS 7 in install

http://tracker.ceph.com/issues/9041
You add a valid point. This will break again with CentOS8/RHEL8.

The fix would involve to detect both `rhel` and `centos` and slap the version to it. But such auto-detection might need extra work to make it non-brittle. For example, if the distro decides not to start the release with say '8' but does something like `"08"` or `" 8"`.

I don't think you trump readability by saying "if item in list" btw. No need for repetition :)

If you are up for it, go ahead in that direction, I would prefer less `if` conditions though