build/ops: rpm: sane packaging of %{_docdir}/ceph directory

build/ops: rpm: sane packaging of %{_docdir}/ceph directory

91648044163 introduced a build error:

ceph-common-12.1.0+git.1498286248.2fcedc7b3d-1.1.x86_64.rpm: directories not
owned by a package:
 - /usr/share/doc/packages/ceph

The %docdir directive is a way of flagging anything in that directory as being
documentation. It does not actually package the directory. And we don't need
it because we're not dumping a large number of files into this directory.
For more information, see the "Directory-related Directives" section of
http://ftp.rpm.org/max-rpm/s1-rpm-inside-files-list-directives.html

Signed-off-by: Nathan Cutler <ncutler@suse.com>