Serve 502 pages

Summary:
As 726fa910fe44 provides a generic 502 page (and a variation for Cachet),
it's now possible to ensure every nginx block has its own 502.

An include isn't optimal, as it does something
not to hide en dessous du tapis: redefine $root.

Ref T114

Test Plan: Run PaaSDockerTest::test502Pages test

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T114

Differential Revision: https://devcentral.nasqueron.org/D1972