This fixes a bug with JSON agent configuration parsing where the AST
for the plugin stanza had unnecessary flattening originating from hcl parsing
library. The workaround fixes the AST by popping off the flattened element and wrapping
it in a list. The workaround comes from similar code in terraform.

There were no existing test cases for json parsing so I added a few.

minor fixes to spread stanza documentation

tests: port pre-0.9 task env tests

I chose to make them more of integration tests since there's a lot more
plumbing involved. The internal implementation details of how we craft
task envs can now change and these tests will still properly assert the
task runtime environment is setup properly.

test: properly skip client state in beforeall

release: skip check step when cutting release

Now `prerelease` no longer requires linters

`make check` runs very intensive linters that slow and seem to behave
differently on different machines.

Linting is still a part of our CI and we shouldn't be cutting a release
when CI isn't green anyway.

client/logmon: restart log collection correctly when a task is restarted

Prometheus metrics for the e2e environment

Test state corruption

The e2e test code is absolutely hideous and leaks processes and files
on disk. NomadAgent seems useful, but the clientstate e2e tests are very
messy and slow. The last test "Corrupt" is probably the most useful as
it explicitly corrupts the state file whereas the other tests attempt to
reproduce steps thought to cause corruption in earlier releases of
Nomad.

docs: sync systemd unit files; update deploy guide

Point users to security doc instead. Right now it takes a lot of
explaining to describe to users exactly how to validate the binary and
what the output of the tools used means.

For example, this is the output when validating according to the
instructions in this guide and the linked doc:

```
vagrant@linux:/tmp$ gpg --verify nomad_0.8.7_SHA256SUMS.sig
nomad_0.8.7_SHA256SUMS
gpg: Signature made Fri 11 Jan 2019 09:47:56 PM UTC using RSA key ID
348FFC4C
gpg: Good signature from "HashiCorp Security <security@hashicorp.com>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the
owner.
Primary key fingerprint: 91A6 E7F8 5D05 C656 30BE  F189 5185 2D87 348F
FC4C
vagrant@linux:/tmp$ shasum -a 256 -c nomad_0.8.7_SHA256SUMS
shasum: ./nomad_0.8.7_darwin_amd64.zip:
./nomad_0.8.7_darwin_amd64.zip: FAILED open or read
shasum: ./nomad_0.8.7_linux_386.zip: No such file or directory
./nomad_0.8.7_linux_386.zip: FAILED open or read
shasum: ./nomad_0.8.7_linux_amd64-lxc.zip: No such file or directory
./nomad_0.8.7_linux_amd64-lxc.zip: FAILED open or read
./nomad_0.8.7_linux_amd64.zip: OK
shasum: ./nomad_0.8.7_linux_arm64.zip: No such file or directory
./nomad_0.8.7_linux_arm64.zip: FAILED open or read
shasum: ./nomad_0.8.7_linux_arm.zip: No such file or directory
./nomad_0.8.7_linux_arm.zip: FAILED open or read
shasum: ./nomad_0.8.7_windows_386.zip: No such file or directory
./nomad_0.8.7_windows_386.zip: FAILED open or read
shasum: ./nomad_0.8.7_windows_amd64.zip: No such file or directory
./nomad_0.8.7_windows_amd64.zip: FAILED open or read
shasum: WARNING: 7 listed files could not be read
```

There are only two lines that matter in all of that output:

```
...
gpg: Good signature from "HashiCorp Security <security@hashicorp.com>"
...
./nomad_0.8.7_linux_amd64.zip: OK
...
```

I feel like trying to teach users how to use and interpret these tools
in our deployment guide may be as likely to reduce confidence as
increase it.

tls_cipher_suites only accept a comma-separated string, as evident in:

https://github.com/hashicorp/nomad/blob/0535dfd414214efd14183bf23d1447f679b57af8/nomad/structs/config/tls.go#L61
https://github.com/hashicorp/nomad/blob/d37ed5c193ac8237666b92bd3f3b0b7e0a8ebfd5/helper/tlsutil/config.go#L405

Co-Authored-By: schmichael <michael.schurter@gmail.com>

docs: add #5381 to changelog

server: inconsistent receiver notation corrected

Reverts hashicorp/nomad#5433

Apparently, channel communications can constitute Happens-Before even for proximate variables, so this syncing isn't necessary.

> _The closing of a channel happens before a receive that returns a zero value because the channel is closed._
https://golang.org/ref/mem#tmp_7

Scripts for automating binary building

To correspond to the Linux scripts in 855761a5

The systemd configs spread across our repo were fairly out of sync. This
should get them on our best practices.

The deployment guide also had some strange things like running Nomad as
a non-root user. It would be fine for servers but completely breaks
clients. For simplicity I simply removed the non-root user references.