Implement checks for CentOS 7 in install

http://tracker.ceph.com/issues/9041
I debated doing it that way, but opted to use the existing syntax that was there to maintain clarity.

To be honest I toyed with the idea of just checking to see if the version was greater than 6, and if it was then construct the url based on which OS.

So something like:

if os version > 5: 
   if os == centos:
      return el<os version>
   elif os = redhat:
      return rhel<os version>
   else
      return 'el6'

What about something like that? That would certain work for all future versions potentially avoid this issue when 8.0 arrives.