osmo-gsm-tester: Pass conf dir through -c cmd line arg instead of env var
As of osmo-gsm-tester.git Change-Id I7eb69f870d0dcb5906d45ae067d6bed1aabf5862, the enviornment variable OSMO_GSM_TESTER_CONF is deprecated in favour of already existing -c cmd line argument. Let's use it while still providing the docker setup with a way (using same env var name) to modify where the conf dir is to be found inside the docker container (for instance, to provide an external conf dir).
