packaging: use dist tag in RPM Release field
packaging: use dist tag in RPM Release field

This change adds ".el6", ".el7", etc to the end of the RPM filename.

More information about the dist tag can be found at
http://fedoraproject.org/wiki/Packaging:DistTag

Signed-off-by: Ken Dreyer <940dddfbf8b0685439b7ecf27daa0ba58b359a2c@redhat.com>

ceph/radosgw-agent,ceph/radosgw-agent