clear mobile-gateway command hierarchy

    clear
      — mobile-gateway
          — bng
              — error-history
              — session
              — session-lockout
              — wpp
                  — statistics
          — pdn
              — bearer-context
                  — apn
                  — imei
                  — imsi
                  — msisdn
                  — up-peer
              — call-flow-stats
              — path-mgmt-stats
              — ref-point-stats
              — statistics