Bring more structure to RU-specific code as a preparatory step for multiRU support:

- move RU-specific files under ru/<RU-type>/ . This mostly moves Lopcomm programs and configuration files there.
- move RU-specific instance code there as well. This also mostly moves Lopcomm specific services and promises there.
- bring more structure in naming. As buildout has global namespace use ru_<RU-type>_ prefix to avoid collision in names.

This should be a preparatory patch with practically no semantic change, but
preparing ground for further multiRU landing.

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu
/reviewed-on nexedi/slapos!1466