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
Change-Id: If75e902b451d7e202a03e93afcd55bd24f517813