Pass user-requested attributes from CLI to whitelist

This greatly speeds up running ohai from the command line when given a
set of attributes to display (like `ohai ec2`). Prior to this patch,
ohai would run all plugins and then filter out the data. With this
patch, ohai only runs relevant plugins.

As an unscientific test `bin/ohai network` on my laptop takes ~4s
without this patch compared to ~1.1s with the patch.