A few fixes to the Ceph repos.

* Ceph Dumpling is the LTS release, it should be used instead of Emperor.
  The next stable/LTS release is going to be Firefly.
* CentOS is now using the CentOS repo instead of RHEL6 repo.
* RHEL is now using the RHEL6.4 repo instead of RHEL6.
* Added the ceph-extras repo for all. This includes needed packages
  that do not come (or are too old) on their specific distro.
* For RPM repos naming has been corrected and priorities added.

Signed-off-by: JuanJose "JJ" Galvez <jj.galvez@inktank.com>