Fixes #3357

interop tests are unhappy:

START: build
WAITING: 1 jobs running, 0 complete, 0 failed
PASSED: build [time=917.0sec]
START: large_unary
START: empty_unary
START: ping_pong
START: client_streaming
START: server_streaming
WAITING: 5 jobs running, 0 complete, 0 failed
ERROR: unknown command line flag 'use_tls'

I imagine it might be picking up some old docker image at some point, which is why we are working on revamping the interop tests.

Please update https://github.com/grpc/grpc/blob/master/src/objective-c/tests/run_tests.sh#L40 too.

Now that #3373 has been merged, can you pls try to rebase -i upstream/master and update the run_interop_tests.py script accordingly? After that the unknown command line flag problem should be gone.

Btw, the spec explicitly requires supporting --use_tls=true (and not just --use_tls), does that work for C++?

Tests seems to pass locally.