Configure feature flags with environment variables (#8256)
Configure feature flags with environment variables (#8256)

Should be useful for benchmarking and other circumstances where
HailContexts are being started and stopped more often.
danking/hail,cseed/hail,danking/hail,cseed/hail,hail-is/hail,cseed/hail,cseed/hail,hail-is/hail,cseed/hail,cseed/hail,danking/hail,hail-is/hail,danking/hail,hail-is/hail,cseed/hail,danking/hail,hail-is/hail,hail-is/hail,danking/hail,danking/hail,hail-is/hail,cseed/hail,hail-is/hail,danking/hail