add a yum clean helper

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
uninstall ceph-release and clean cache in centos

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #163 from alfredodeza/7390

remove ceph-release and call yum clean all