The “Facts of Life” cast for “Live in Front of a Studio Audience” special has been announced: Jennifer Aniston will play Blair . . . Gabrielle Union will play Tootie . . . Kathryn Hahn will play Jo . . . and Allison Tolman is Natalie.  Ann Dowd will play Mrs. Garrett.  AND  “Live in Front of a Studio Audience:  Diff’rent Strokes’” will air live Tuesday, Dec. 7 at 8 p.m. ET on ABC