Add codec parameter to start_call() and convenience functions mt_call()
and calls() with FR as default. While at it, update the help text.
Duplicate enum codec_type into mncc_mt_loadgen.py, because the enum
values are different from the existing enum values in the python code
(GSM48_BCAP_SV_EFR etc.).
Related: SYS#4924
Instead of hardcoding FR in rtpsource, add an argument to set the codec
from mncc_mt_loadgen.py via CTRL call rtp_create. Hardcode FR in
mncc_mt_loadgen.py for now, a follow up patch will make it configurable
there, too.
Related: SYS#4924
osmo-rtpsource is a small utility program which is generating
RTP flows at a constant rate of 20ms, as required in most IP based
telphony. The payload currently is hard-coded.
Change-Id: Id4e292ddfd5aa58754382b2380558993b2ddf07a