Set GPU frequencies via NVML

Add step_configure_hardware() to GRES plugin API.
Call step_configure_hardware() from stepd and make sure it's root.
Sort frequencies returned by NVML.
Update GRES-related docs.
Document step_configure_hardware() and get_devices().
Test the bubble sort algorithm.
Update test 39.9 to work with new code.
Remove vestigial references to volts in the docs.