check all the docker images before starting the test

[tarekziade]

Right now if a docker image is missing, you're getting the problem when coreos tries to pull it.

[2016-12-02 09:54:38,100][68372] Exception with run_container: 404 Client Error: Not Found ("b'No such image: kinto/loadtest:latest'")

To avoid any work and time wasted we should check upfront that all the required resources to run a test exist. That will avoid running amazon boxes, and dockers and fail miserably 10 minutes later :)