loadgenerator: switch to python 3.6

python3.7 is causing lockup in locust.
See https://github.com/locustio/locust/issues/843

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

src/loadgenerator/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3
+FROM python:3.6
 
 COPY requirements.txt .
 RUN pip install -r requirements.txt

src/loadgenerator/loadgen.sh

@@ -7,5 +7,4 @@ if [[ -z "${FRONTEND_ADDR}" ]]; then
 fi
 
 set -x
-# add "timeout 3600" because locust locks up/freezes for some reason
-timeout 3600 locust --host="http://${FRONTEND_ADDR}" --no-web -c "${USERS:-10}" -r 10 
+locust --host="http://${FRONTEND_ADDR}" --no-web -c "${USERS:-10}"