cephadm_adopt: fetch and write ceph minimal config

cephadm_adopt: fetch and write ceph minimal config

This commit makes the playbook fetch the minimal current ceph
configuration and write it later on monitoring nodes so `cephadm` can
proceed with the adoption.
When a monitoring stack was deployed on a dedicated node, it means no
`ceph.conf` file was written, `cephadm` requires a `ceph.conf` in order
to adopt the daemon present on the node.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1939887
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>