Allow mcollective::common::setting to be overridden

mcollective::common::setting was intended to follow the same pattern as
mcollective::client::setting and mcollective::server::setting, but it was
overlooked when initially written.

This patch enables this pattern as was originally intended:

    mcollective::common::setting { 'daemonize':
      value => 1,
    }

    # at some distance
    mcollective::common::setting { 'override daemonize':
      setting => 'daemonize',
      value => 0,
      order => 90,
    }