Switch from supervisord to systemd

[craigday]

No description provided.

[brikis98]

Thanks for the PR!

A few thoughts:

1. How did you test this?
2. Could you update the example Packer templates to use Amazon Linux 2?
3. Could you update the docs to talk about systemd instead of supervisord?
4. Remove sudo mkdir -p "$path/log" from the install script if we're not writing logs there anymore.
5. Make stdout/stderr settings configurable.
6. Please make sure to enable the service in systemd as shown in Enable systemd consul service terraform-aws-consul#135.

See https://github.com/hashicorp/terraform-aws-consul/pull/130/files for reference of a similar systemd switch that has examples of the above.

[hashicorp-cla]

All committers have signed the CLA.