Custom backend loadtests

This PR adds deployment profiles "loadtest-mysql" and "loadtest-cassandra", for easily trying out different backends.  They're currently very naive and bare-bones, but we can iterate on them with this infra in place.  They require awsboxen 0.1.1, which lets you build AMIs from a shell script.

The loadtest/run.js script is updated to use these profiles for its deployment, allowing you to switch between them with an environment variable.  Running::

    SYNCSTORE_BACKEND=mysql ./loadtest/run.js

Will deploy and test the mysql-backed setup, while::

    SYNCSTORE_BACKEND=cassandra ./loadtest/run.js

Will deploy and test the cassandra-backed setup.

@dannycoates r?