warn on unknown configuration keys in ceph.conf

[alfredodeza]

Since Luminous, it is possible to ask Ceph what are all the configuration keys that are supported. E.g.:

$ ceph --admin-daemon /var/run/ceph/ceph-osd.3.asok config help | grep '"name"'
        "name": "host",
        "name": "fsid",
        "name": "public_addr",
        "name": "public_bind_addr",
        "name": "cluster_addr",
        "name": "public_network",
        "name": "public_network_interface",
        "name": "cluster_network",
        "name": "cluster_network_interface",
        "name": "monmap",
        "name": "mon_host",
        "name": "mon_dns_srv_name",
        "name": "lockdep",
        "name": "lockdep_force_backtrace",

Non-existent variable names should be a warning, it might even be useful to do an approximation of names and suggest a possible (correct) name.