This information about the Homebrew user was needed for a workaround
which involved invoking Homebrew directly,
since Homebrew cannot be run as root.

However, the improved Homebrew versions support means this workaround
(to installing autoconf and autoconf@2.13 concurrently)
is no longer necessary and was removed in
commit a063248.

This code was inadvertently left dangling, and is now cleaned up.

Fully managing the contents of the whole hosts file ensures that unused
or obsolete hosts entries are removed promptly.

Since we have setup DNS for our build machines,
remove the relevant static entries from the hosts file.

These states should only be run for fully managed nodes,
e.g. those under our administrative control,
so move them to the admin/init.sls file.

This is part of a refactor making it possible to run the states
installing Salt build dependencies without making other system level
changes, for use in mach bootstrap.

Use the `os` grain instead of the `kernel` grain for targeting,
for slightly better semantics.

Move the Python (2 and 3) related states into a separate SLS file,
so they can be reused without pulling in the servo user state,
which is in common.

Instead of adding this new SLS file to the top file,
include it from other SLS files that depend on a Python installation.

This makes it possible to `state.apply servo-build-dependencies`
independently, with is needed for `mach bootstrap`.

Note: No effort was made to separate Python 2 and Python 3,
as the virtualenv pip package will create executables for both.

The reference to /etc/apt/sources.list.d allows cleaning that directory
of extraneous sources files. This should only be done on fully managed
nodes, so guard this requisite with a pillar check.

The check defaults to True for backwards compatibility
(i.e. not having to change the production pillars).

This will make it easier to maintain these lists.
Additionally, be more strict about matching on OS type
to make it easier to support more OSes in the future.