dhcp: multiple additions

DHCP additions:
1) DHCPv4 will only relay a message back to the client, if the Option82 information is present. So make this the default.
2) It is no longer possible to select via the API to "insert circuit ID" - since this is now default
3) Remove the version 2 API since it's now the same as version 1.
4) Adding the VSS option is now conditional only on the presence of VSS config (not the 'insert' option in the set API)
5) DHCP proxy dump via API

Change-Id: Ia7271ba8c1d4dbf34a02c401d268ccfbb1b74f17
Signed-off-by: Neale Ranns <nranns@cisco.com>