ceph backend packaging dependencies

[dotnwat]

there are two components to the ceph backend:

1. libcls_zlog
2. libzlog_ceph_backend

The libcls_zlog depends on the version of the OSD that is deployed, while libzlog_ceph_backend only depends on librados. Users upgrade OSDs and need new libcls_zlog versions, while users of librados simply need wire compatibility.

libcls_zlog

We'll build frequent versions of libcls_zlog against various versions of upstream Ceph releases.

libzlog_ceph_backend

Some distributions (e.g. jessie, trusty, centos7) have versions of librados that are too old to talk to luminous. They might be fine for some users running older Ceph clusters. So we should do the following:

1. build libzlog_ceph_backend packages for librados in distribution repositories. users are responsible for knowing if the version is ok for their cluster.
2. make sure that source versions install well for users that need to compile against newer versions of librados
3. release packages for specific releases of ceph as requested.