OpenCL runtime uses device specified by HL_GPU_DEVICE

This is a rather trivial pull request, I wanted to give @jrprice (#326) a chance to review the environment variable name changes. I'm not sure if they're used anywhere yet.

For what it's worth, my philosophy on this is that any application that hopes to provide cross platform OpenCL support will need to provide its own halide_acquire_cl_context with more interesting logic. All our platform/device environment variables for OpenCL are really only useful for testing/development IMO. They won't be reliable across machines or even possibly other more specific circumstances.