Add support for parameterized load test module files

[pdehaan]

Something like this in a Dockerfile:

CMD URL_SERVER=$URL_SERVER molotov -c $VERBOSE -p $TEST_PROCESSES -d $TEST_DURATION -w $TEST_CONNECTIONS ${TEST_MODULE:-loadtest.py}

Re: https://docs.docker.com/engine/reference/builder/#environment-replacement

The ${variable_name} syntax also supports a few of the standard bash modifiers as specified below:

• ${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.

So in our case we can pass a custom value for TEST_MODULE if we need it (ie: loadtest-read.py), but by default it will use loadtest.py.

[tarekziade]

you can define all the running options in the top-level molotov.json file in your repo. see http://molotov.readthedocs.io/en/latest/slave/

from there, the cmd docker is running is 'moloslave', which pulls the repo and run the test with the json file.