Changes

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).

Change-Id: I54ea9dc4d0bb205debf312f5a37053bc60cfcdb7