Refs #3809 - Turning on some rubocop cops

fixes #14661 - support IPv6 subnets

fixes #14638 - Refactor Subnet into STI to allow different subnet types

Fixes #13725 - DHCP MAC conflicts are reported

Refs #12911 - Fixing colon method calls

fixes #12893 - ensure constants are set only once

the root cause is probably the same file is loaded multiple times.

Fixes #11709 - better error for DNS conflict timeouts

Refs #3809 - Remove semicolon cop

fixes #10020 - add next-server/filename attrs to provision NIC DHCP record

A host's DHCP record used to be constructed with PXE attributes in
Orchestration::DHCP, but moved to Nic::Managed during 43c4bd7. Nic::Managed
didn't add PXE attributes, so this commit adds these when provision? is set....

Fixes #8727 - DHCP validation does not fail on discovered leases