If we have several radio units, we want each of them to have its own sdr-busy promise.

See nexedi/slapos.toolbox!125 for
corresponding part in slapos.toolbox that makes check_sdr_busy per-RU
configurable.

Not touching gnb.jinja2.cfg as currently gnb does not support multicell at all,
and in the future it will be sole enb.cfg to handle both LTE and NR
simultaneously.

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu
/reviewed-on nexedi/slapos!1485