Be explicit about legacy var additions/omissions

Behavior changes between v2 environment variables and the v3
zuul_legavy_vars filter may cause regressions in some translated
jobs, so be very explicit about which variables were intentionally
omitted, which variables are completely new, and which variables are
now appearing in pipelines they did not previously.

Change-Id: Idf032f9903b21e383a733ab9ff1a6ec80f636e4a
1 file changed