cephadm-adopt: ensure /etc/ceph is present on monitoring node

cephadm-adopt: ensure /etc/ceph is present on monitoring node

When deploying the monitoring stack on a dedicated node, the directory
`/etc/ceph` has never been created. Therefore, the play for adopting the
monitoring stack fails because it can't write the minimal config file.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2029697
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 7ece59b41defc81ffd3bf184a24b63b45ec7d097)
(cherry picked from commit 8a32576d209bc329b4159a781151a7d3a40c5b7d)