Description of problem:
RHEL 7.4 includes ceph-common shipped in the base server repository.  This seems to be from ceph 2.0 and generates package conflicts when installing ceph 1.3 content.

Basically, the issue is that ceph-osd, ceph-mon, etc all require ceph which requires ceph-common, all of which are the same version.  This expands to the other ceph-common packages like librados2 and librbd1.  An example of this is below.

Note:  it's possible to yum install if there are *no* ceph packages pre-installed, but yum updates would fail because it would try to update the ceph-common packages.  

Example:

Error: Package: 1:python-rados-0.94.9-9.el7cp.x86_64 (rhos-9.0-director-ceph-1.3-mon-signed)
           Requires: librados2 = 1:0.94.9-9.el7cp
           Installed: 1:librados2-10.2.5-3.el7.x86_64 (@rhos-9.0-rhel-7-signed)
               librados2 = 1:10.2.5-3.el7