Abstract host release/distro detection 

This paves the way for a better abstraction to host detection (ticket http://tracker.ceph.com/issues/5727). It removes all of the install functions from `ceph_deploy/install.py` into their own distro module which in turn they get imported depending on the host distribution.

The code will become immediately more readable as things are completely modularized now and now special casing will be needed to take an action.