cephadm: use dashes for container names

cephadm: use dashes for container names

podman adds the current container name to the /etc/hosts
file. Turns out, python's `socket.getfqdn()` differs from
`hostname -f`, when we have the container names containing
dots in it.:

[root@sebastians-laptop /]# cat /etc/hosts
127.0.0.1   localhost
::1         localhost
127.0.1.1   sebastians-laptop foo.bar.baz.com
[root@sebastians-laptop /]# hostname -f
sebastians-laptop
[root@sebastians-laptop /]# python3 -c 'import socket; print(socket.getfqdn())'
foo.bar.baz.com

Fascinatingly, this doesn't happen when using dashes.

Fixes: https://tracker.ceph.com/issues/51590
Signed-off-by: Sebastian Wagner <sewagner@redhat.com>