DHCP Multiple Servers (VPP-602, VPP-605)

DHCP Multiple Servers (VPP-602, VPP-605)

Multiple DHCP (4 and/or 6) servers can be added and removed through multiple calls to the 'set dhcp server' API.
All 4/6/ discover/solicit messages will then be replicated to all servers in the list. The expectation is that the servers/system is configured in such a way that this is viable.
If VSS information is providied for the clinet VRF which also has multiple servers configured, then the same VSS information is sent to each server. Likewise the source address of packets sent to from VPP to each server is the same.

Change-Id: I3287cb084c84b3f612b78bc69cfcb5b9c1f8934d
Signed-off-by: Neale Ranns <nranns@cisco.com>