ceph.spec.in: move "Requires: python3-setuptools" from ceph-base to ceph-osd

ceph.spec.in: move "Requires: python3-setuptools" from ceph-base to ceph-osd

python3-setuptools was originally added to ceph-base as a dependency of
ceph-detect-init, see https://tracker.ceph.com/issues/14864. but since
ceph-disk and ceph-detect-init were replaced by ceph-volume, and were
removed from the debian packaging in
ee6bc23e892369b14668faea04c3ef1b5776a6b6.

there is no need to have python3-setuptools in the ceph-base packages
anymore.

but since we are still using pkg_resources module provided by setuptools
in ceph-volume, we need to preserve this runtime dependency in ceph-osd.
as ceph-osd packages ceph-volume.

please note, pkg_resources module is also used by cephadm to poke around
ceph_iscsi python module installed in a container, so python-setuptools
should be installed along with ceph-iscsi if we need a better
interoperability between ceph-iscsi and cephadm. this is not in the
scope of this change.

Signed-off-by: Kefu Chai <kchai@redhat.com>