refactor: generalize Distro add_user and shutdown_command

[blackboxsw]

Decomposed #6689 into part-1 and part-2 to aid reviewing.

Proposed Commit Message

Perform preliminary refactor to be used by securirity event logging.

Split add_user method into separate methods:
- _add_user_preprocess_kwargs: filter distro-specific args before cmd
- _build_add_user_cmd: return tuple of cmd and log_command for the useradd
- _post_add_user: distro-specific post-creation steps for Alpine
- _user_groups_to_list: normalize group input to a list

Move util.is_user check into create_user and make add_user raise on failure instead of returning bool.  Subclasse now only override the separate methods instead of duplicating add_user.

Refactor shutdown_command introducing a new _build_shutdown_command which is overridden in subclasses.

Additional Context

Test Steps

tox -e py3

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)