Update loadtests to hit the raw_password API.

This updates the python loadtests to use a 50/50 split between srp auth and raw-password auth.  Unfortunately we can't check for cross-auth-style compatability, since that would mean we have to do keystretching as part of the loadtest.

I have **not** updated the javascript tests to match.  Should I?  @jbonacci what do you think about the relative merits of python tests vs javascript tests at this point in development?

Also, @jbonacci r?