Rework UEsim to be able to work with multiple cells, multiple radio units(*),
multiple UE all at the same time. RU, CELLs and UEs are now configured,
simiularly to eNB, via shared instances. Add tests.

Contrary to ORS don't care about backward compatibility here because currently
we have just a few UEsim deployments and migrating them should be easy.

Please see added schemas, tests and updates slapos-render-config on how to use
the new system.

(*) contrary to eNB UEsim does not allow to use one RU for two cells,