debian: use python:Depends for ceph-{common,base,osd}'s deps

debian: use python:Depends for ceph-{common,base,osd}'s deps

* ceph-base: use ${python:Depends} instead of listing the python
  dependencies manually, dh_python2 will scan the requirements
  of ceph-detect-init. and fill the subst var for us.
* ceph-common: add ${python:Depends}, as it packages ceph,
  and ceph-brag client.
* ceph-osd: it packages ceph-disk, so should add ${python:Depends}
  as its dependencies.

dh_python2 will figure them out.

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