grpc stress tests transcode and pass_through evently.
grpc stress tests transcode and pass_through evently.

grpc_pass_through test has 12 iterations with different
parameters.  Previous change run one iteration of pass
through test and run a full transcoding test. It is too
much of transcoding test.
This change will run all iterations of pass_through tests
then transcoding test for each long-run iteration.

Change-Id: I3e59575073dbc7aec9cf47b39a5bd6723ca90487

cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp