[2.0] dhcp6 configuration is broken: expects CIDR not netmask

[LaMont Jones]

When a dhcp-enabled ipv6 subnet exists, dhcpd complains thusly:

Apr 1 05:46:10 kohlrabi dhcpd[25894]: Copyright 2004-2015 Internet Systems Consortium.
Apr 1 05:46:10 kohlrabi dhcpd[25894]: All rights reserved.
Apr 1 05:46:10 kohlrabi dhcpd[25894]: For info, please visit https://www.isc.org/software/dhcp/
Apr 1 05:46:10 kohlrabi dhcpd[25894]: /var/lib/maas/dhcpd6.conf line 36: Expecting a '/'.
Apr 1 05:46:10 kohlrabi sh[25894]: /var/lib/maas/dhcpd6.conf line 36: Expecting a '/'.
Apr 1 05:46:10 kohlrabi sh[25894]: subnet6 2601:282:8100:3580:: netmask
Apr 1 05:46:10 kohlrabi sh[25894]: ^
Apr 1 05:46:10 kohlrabi sh[25894]: /var/lib/maas/dhcpd6.conf line 54: empty shared-network decl
Apr 1 05:46:10 kohlrabi sh[25894]: }
Apr 1 05:46:10 kohlrabi sh[25894]: ^
Apr 1 05:46:10 kohlrabi dhcpd[25894]: subnet6 2601:282:8100:3580:: netmask
Apr 1 05:46:10 kohlrabi systemd[1]: maas-dhcpd6.service: Main process exited, code=exited, status=1/FAILURE
Apr 1 05:46:10 kohlrabi sh[25894]: Configuration file errors encountered -- exiting

The configuration of subnet6 expects a CIDR, rather than network and netmask.