tests: ceph_objectstore_tool.py main returns

Instead of calling sys.exit() the main function returns the desired exit
code.

Signed-off-by: Loic Dachary <ldachary@redhat.com>
tests: ceph_objectstore_tool.py encapsulate init-ceph stop

Call init-ceph in kill_daemons and add a call to kill_daemon when main
returns on error so that it never leaves daemons hanging.

Signed-off-by: Loic Dachary <ldachary@redhat.com>