osd: commonize start_osd code

osd: commonize start_osd code

since `ceph-volume` introduction, there is no need to split those tasks.

Let's refact this part of the code so it's clearer.

By the way, this was breaking rolling_update.yml when `openstack_config:
true` playbook because nothing ensured OSDs were started in ceph-osd role (In
`openstack_config.yml` there is a check ensuring all OSD are UP which was
obviously failing) and resulted with OSDs on the last OSD node not started
anyway.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>