Lopcomm part of the SR already has some partial support for cell_list: if
multiple cells are defined there enb.cfg will have multiple CPRI radio units
and multiple cells configured. But so far all promises, except cpri-link, were
done only for one RU.

-> Fix that by starting to generalize RUlib code to handle multiple radio
units, invoking model-specific RU driver for each RU, and adjusting ru/lopcomm
instance code to correctly generate and activate different promises for
different radio units.

After the patch multiRU support is still very incomplete, but it is a step forward.

/cc @xavier_thompson, @Daetalus
/reviewed-by @lu.xu
/partly-reviewed-by @jhuge
/reviewed-at nexedi/slapos!1467