Use last address in v6 allocation pool generation

Use last address in v6 allocation pool generation

This reverts commit 1d61c5f736b9df7253a1939fe55f1875f988d1f4,
which reverted ee51ef72d37a02005a7733b7f2faf7236db850a1 due
to an issue with the allocation pools being mis-aligned with
the last address of a v6 subnet.

IPv6 does not have a subnet broadcast address so the last IP
address can be used as a regular address. This patch tweaks the
automatic allocation pool generation to include that last address
on v6 networks.

This was found because
NetworksTestDHCPv6.test_dhcp_stateful_fixedips_outrange makes the
assumption that the default generated pools cover the entire usable
address space.

Related-Bug: #1466322
Closes-Bug: #1468163
Change-Id: I5a2518c819757cd1fb43244901fe3e2f6c576b34