[RFE] distro-specific groups in functional tests

## Description

https://github.com/Azure/azure-init/pull/125 hot-fixed [boot failure](https://github.com/Azure/azure-init/issues/124) of functional tests with Debian-derivatives.

However, there are still remaining issues around functional tests to support various cases for the default user group. Following places do not care about groups.

* The systemd unit [azure-init.service](https://github.com/Azure/azure-init/blob/e064a84c7203f1e43a07cdc5556f14aca5a6db58/config/azure-init.service#L5) invokes azure-init binary without specifying custom groups with `--groups`.
* [functional_tests.rs](https://github.com/Azure/azure-init/blob/e064a84c7203f1e43a07cdc5556f14aca5a6db58/tests/functional_tests.rs#L86).

Ideally we should have general templates for the systemd service and group lists for `with_groups` in functional tests. Those should be converted into distro-specific files that can be used for build system. One option would be to use [askama](https://github.com/djc/askama), a Jinja-like templates for Rust.

See also https://github.com/Azure/azure-init/issues/106.

Start by reading config/azure-init.service and tests/functional_tests.rs, then review PR 125, issue 124, and issue 106 for the existing Debian-derivative workaround and remaining group cases. Define how distro-specific service templates and functional-test group lists should be represented and consumed by the build system; done means the relevant functional tests support the documented distro cases.

Written by the indexing model from the issue text.