It should be possible to use the Docker image directly as `docker run` instead of `local.sh` about docker-locust

[scherniavsky]

Added ROLE=standalone that spins up controller, master and slaves in the same container.
E.g.:

• Automatic testing of example.com:
  docker run -i -v ./example:/opt/script -e TARGET_HOST=http://example.com -e LOCUST_FILE=simple.py -e ROLE=standalone -e AUTOMATIC=1 -e DURATION=10 -e HATCH_RATE=1 -e USERS=0 docker-locust

• Automatic testing of localhost:
  docker run --net=host -i -v ./example:/opt/script -e TARGET_HOST=http://localhost -e LOCUST_FILE=simple.py -e ROLE=standalone -e AUTOMATIC=1 -e DURATION=10 -e HATCH_RATE=1 -e USERS=0 docker-locust

• Manual testing of localhost:
  docker run --net=host -i -v ./example:/opt/script -e TARGET_HOST=http://localhost -e LOCUST_FILE=simple.py -e ROLE=standalone -e DURATION=10 -e HATCH_RATE=1 -e USERS=0 docker-locust