feat(dracut): support parallel execution with --parallel

feat(dracut): support parallel execution with --parallel

Add an option --parallel which can currently be used with
--regenerate-all. With --regenerate-all --parallel, dracut will be
run in parallel for all kernels found.

Also introduce a config file equivalent setting: parallel="yes".
Making this work requires moving the code block that handles --regenerate-all
behind the code block for reading the config files.

Signed-off-by: Martin Wilck <mwilck@suse.com>