Adds a network stanza and additional options to the task group level
in prep for allowing shared networking between tasks of an alloc.

command/agent: allow the job GC interval to be configured

This addresses the issue raised by @pznamensky in #5966.

docs: job names are unique by region/namespace

Revert "driver/docker: support unix destination mount path in windows"

docs: mention acls & sentinel are multi-region

* fix navigation issue for spread guide

* skeleton for preemption guide

* background info, challenge, and pre-reqs

* steps

* rewording of intro

* re-wording

* adding more detail to intro

* clarify use of preemption in intro

driver/docker: support unix destination mount path in windows

The reference architecture neglected to mention ACLs and Sentinel
policies are the exception to "no data is shared between regions."

"globally" is incorrect here unless you're running 1 region or at least
only 1 per planet.

Sorted link list at the bottom as well.

Thanks to @notnoop for letting me know about this:
https://github.com/hashicorp/nomad/pull/6007#discussion_r308052454

Clarify encryption keys are shared across datacenters

use go1.11 in windows CI

#5973

document cgroup raw exec

Support for hot reloading log levels

This is an extraneous addition from #4612.

This also includes migration of some tests to async.