Convert zuul.projects to a dict

This follows-on from I4476b9d4915d107e29b91229287865bff0ada305 where
we are converting zuul.projects to a dict for easier access.

With the dependent changes, there should be no in-tree users of
zuul.projects so we can do this switch.  We will then convert
zuul._project users back to zuul.projects for the final removal
(I283e66293110aa3bc99acb1cbc6eb3e0cc11f750).

This updates documentation, and also gives some samples of how to use
the variables (the "| list" is a bit of a gotcha trick -- python3's
values() returns a view, so it is necessary for iteration).

Depends-On: Id9a7c137ca5bed25d81087201091157c8401576a
Depends-On: I9d88f405f34d1c5f75ebf4f52cedfaaab20c3bda
Change-Id: I3c011f72933e98ccbf8badf0e9197c8659766c51
2 files changed