call_helpers: Make default behaviour of subprocess_run compatible

call_helpers: Make default behaviour of subprocess_run compatible

subprocess_run just calls call_and_capture which has different default
settings (return text instead of bytes, split lines).
However, subprocess_run should be a drop-in replacement of the python3.5
subprocess.run function, so should behave the same way.

This little trick fixed e.g. calls to "get_cnf -j" called from cnfvar.py
on python 3.3 (current intra2net system version)