This is preparatory step for multiRU: when there will be several LTE cells,
each possibly having different RF mode, we'll need to configure DRB per-cell.

-> Move DRB configuration to separate jinja2 template to prepare to handle that.

This is 99% movement only, without changing the code for DRB profile. We'll
adjust the DRB profile a bit as another preparatory step in the next patch.

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu
/reviewed-on nexedi/slapos!1473