dhcp6: Add new duid class DUID_UUID

This complies to RFC 6355 [1] which defines DUID-UUID Format
as client identifier as:

DUID-Type -  DUID-UUID (4) - (16 bits)
UUID -  An [RFC4122] UUID (128 bits)

NetworkManager in latest ubuntu uses this format and without
this class we cannot parse the DHCPv6 request in order to
get the client id.

[1] http://tools.ietf.org/html/rfc6355