fixes #17866 - Remove PXE Properties from Gentoo Operatingsystem

The Gentoo Operatingsystem cannot be installed via PXE and can only be provisioned via Image.

Implementing the pxe method causes the exception as described in #[17866](http://projects.theforeman.org/issues/17866)....

fixes #16792 - don't modify AR record attributes in-place

Rails 5 may call #freeze on strings passed in as an attribute during
validation, so attributes must not be edited in-place with gsub! and
similar methods.

fixes #17354 - test and improve idempotency of OS fact parser

Fixes #17297 - added PXE loader flags for Debian family

Fixes #16318 - PXE templates now work in safemode (#3783)

pxe_kernel_options added to safemode

Fixes #12634 - New HW Model flag pxe_loader

Implementation of

https://github.com/theforeman/rfcs/blob/master/text/0001-PXE-Booting-UEFI.md

In short, new host/hostgroup flag PXE loader is added to specify TFTP
DHCP filename and Foreman now orchestrates all PXE-capable templates...

Refs #3809 - Turning on some Lint cops

Refs #3809 - Turning on some rubocop cops

fixes #9782 - implement mediapath for suse/yast

fixes #12962 - Add Xenserver to list of supported OS families.

Templates are included in community-templates repo.