osds: do not set docker_exec_cmd fact

osds: do not set docker_exec_cmd fact

in `ceph-osd` there is no need to set `docker_exec_cmd` since the only
place where this fact is used is in `openstack_config.yml` which
delegate all docker command to a monitor node. It means we need the
`docker_exec_cmd` fact that has been set referring to `ceph-mon-*`
containers, this fact is already set earlier in `ceph-defaults`.

By the way, when collocating an OSD with a MON it fails because the container
`ceph-osd-{{ ansible_hostname }}` doesn't exist.

Removing this task will allow to collocate an OSD with a MON.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1584179
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>