Allow monitor ssh-parameters to be modified at runtime

The MariaDB monitor variables ssh_ (ssh_user, ssh_keyfile, etc) cannot be set at runtime. This causes problems when config sync is enabled and the maxscale.cnf is ignored.

{

    "errors": [

        {

            "detail": "mariadbmon: The parameter 'ssh_keyfile' cannot be modified at runtime."

        }

    ]

}