quilt-tester: Remove references to provider specific functions
quilt-tester: Remove references to provider specific functions

Before, quilt-tester attempted to protect against leaked instances by
using provider-specific functions to ensure machines are stopped. This
does not offer particularly good protection because the Quilt cluster
uses the same API calls.

A better solution would be to have a completely separate script that
checks for leaked instances. For now, we remove the provider-specific
functions for simplicity, as it allows us to easily extend quilt-tester
to support providers other than Amazon.

LuiseV/quilt,kklin/kelda,kayousterhout/quilt,ejj/quilt-dev,kklin/quilt,kklin/quilt,kklin/kelda,kelda/kelda,LuiseV/quilt,NetSys/quilt,kelda/kelda,quilt/quilt,LuiseV/quilt,ejj/quilt-dev,kklin/kelda,kayousterhout/quilt,LuiseV/quilt,NetSys/quilt,NetSys/quilt,ejj/quilt-dev,NetSys/quilt,LuiseV/quilt,quilt/quilt,kklin/quilt,kelda/kelda,kklin/kelda,kayousterhout/quilt,ejj/quilt-dev,NetSys/quilt,kayousterhout/quilt,kklin/quilt,quilt/quilt,kelda/kelda,quilt/quilt