Updating ceph repository as ceph only keeps the latest version of octopus in...

Updating ceph repository as ceph only keeps the latest version of octopus in https://download.ceph.com/rpm-octopus/el7/x86_64/

continuousintegration/docker/ctafrontend/cc7/etc/yum.repos.d-public/ceph.repo

 [ceph]
 name=Ceph
-baseurl=https://download.ceph.com/rpm-octopus/el7/x86_64/
+baseurl=https://download.ceph.com/rpm-15.2.15/el7/x86_64/
 gpgkey=file:///etc/pki/rpm-gpg/RPM-ASC-KEY-ceph
 gpgcheck=1
 enabled=1