Bug 1045966 — Add a tool that let us validate the Service Provider.

https://bugzilla.mozilla.org/show_bug.cgi?id=1045966
We could add a parameter `--login-endpoint` that takes the Service Provider login endpoint but we will sometimes need to add a --data parameter.

Then the command will looks like:

    python roundTrip.py \
        -H https://msisdn.services.mozilla.com \
        -c 310 -n +1xxxxxxxxxxx --audience app://loop.services.mozilla.com \
        --login-endpoint https://loop.services.mozilla.com/registration \
        --data '{"simplePushURL": "http://httpdeny.org"}'