Docs: add a :default: argument to zuul:attr

This lets us specify the default as part of the "signature" for
each of the attributes.  This helps standardize information about
default values and avoid needing to incorporate it into the text
description of each attribute.

Standardize on lowercase representation of booleans.

Change-Id: I4892b579545dd459fa877feb3014c4c4f2ced2eb
2 files changed