Add -S <system_description> override.

Add -S <system_description> override.

vyatta@vDUT:~$ show lldp neighbors detail
-------------------------------------------------------------------------------
LLDP neighbors:
-------------------------------------------------------------------------------
Interface:    eth0, via: LLDP, RID: 14, Time: 0 day, 00:02:03
  Chassis:
    ChassisID:    mac 00:0c:29:8c:53:7c
    SysName:      R1
    SysDescr:     Vyatta 3500 series running on VSE6.0
    Capability:   Bridge, off
    Capability:   Router, on
    Capability:   Wlan, off
  Port:
    PortID:       ifname eth1
    PortDescr:    vmnet1
    PMD autoneg:  supported: yes, enabled: yes
      Adv:          10Base-T, HD: yes, FD: yes
      Adv:          100Base-T, HD: yes, FD: yes
      Adv:          1000Base-T, HD: no, FD: yes
      MAU oper type: 1000BaseTFD - Four-pair Category 5 UTP, full duplex
mode
  VLAN:         101 eth1.101