cephadm: Give better access to the /dev in the iscsi container

cephadm: Give better access to the /dev in the iscsi container

In testing it seems the main iscsi container's /dev related volume mount to just
/dev/log is too narrow. And in certain circumstances it needs to access
to see /dev/rbd* devices. Like if using krdb.

This patch volume mounts /dev rather then /dev/log in the main
container, but since this aligns with what we need in the tcmu-runner
container it actaully ends up simplifying the code as well.

Signed-off-by: Matthew Oliver <moliver@suse.com>