[inp] teasing apart cookbook upload concerns
Eliminate maybe_* variables
Fill out pending metadata parsing error specs
Fill out pending metadata value specs
Clarify purpose of redis-test-vN tarballs
Document the metadata attributes

Virtus and Yardoc don't play too well together, and Virtus might not be
a known dependency for all contributors, so we wanted to make sure it
was clear why we were using Virtus and how to use it.