Ensure that we're reusing the base configuration between client and
servers without the possibility of drift. Reduce the amount of `sed`
mangling of the configuration file, and make recommended changes from
`shellcheck` for this section of the provisioning script.
Fixes some rebase errors on the Nomad config as well.
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.
