Rename 'recipe' to 'task'

The word 'recipe' is used by other tools (e.g., Chef) for things that
aren't quite the same as what we are talking about in the context of Razor;
to avoid confusion, we are renaming recipes to the more generic word
'task', which still captures that this is about performing some sort of
work against nodes.

This is a follow-on to the renaming of installers to recipes in commit
a9f81b5f and mirrors that closely.