Setting lease time via client classification for ipv6

Setting lease time via client classification for ipv6:

In order to grant the same lease time to a group of clients it would be useful to be able to adjust the lease time via the client classification mechanism. By allowing definition of "valid-lifetime" parameter.
This is a follow on to GL #1635 (closed) "Setting lease time via client classification" which implemented this change for dhcpv4. To my knowledge this has as yet not been request by anyone as of yet.