Handle multivalue DHCP responses

Ticket: ONIE-3
Testing Done:

Tested by sending multivalue DHPC responses, such as:

  option www-server higgs, 10.0.1.51, 10.0.1.49;
  option domain-name-servers 10.0.1.2, 93.187.293.102;

The updated code can consume any multi-value dhcp response correctly.

The following codes could have multiple values:

 3  -- router      -- handled correctly by DHPC response script
 6  -- dns         -- handled correctly by DHPC response script
 7  -- log server  -- currently not implemented
42  -- NTP         -- currently not implemented
72  -- www         -- handled correctly by http waterfall
150 -- tftp server -- currently not implemented