ceph.spec.in: add ceph-mgr package

ceph.spec.in: add ceph-mgr package

This adds ceph-mgr following the same pattern used for ceph-mon,
ceph-mds, ceph-osd.  I've removed the boost-python BuildRequire
for SUSE distros, because we don't ship that (the relevant bits
are already included in boost-devel).

NOTE: This gives an installable ceph-mgr, but not necessarily a
runnable ceph-mgr.  This is because ceph-mgr needs the following
python packages:

- CherryPy
- Django
- gevent
- python-dateutil
- djangorestframework

Not all of these are available as distro packages.  For example,
on openSUSE Tumbleweed, we don't have djangorestframework available
in the base distro, and our Django version is 1.9, which seems to
be incompatible with ceph-mgr.  I'm not sure of the situation on
other distros.

Signed-off-by: Tim Serong <tserong@suse.com>