[Isolate] Serve discovery document over webapp2 routes
[Isolate] Serve discovery document over webapp2 routes

endpoints_webapp2.api_routes installs just the service's handlers.
endpoints_webapp2.api_server also installs discovery handlers.

Bug: 805543
Change-Id: I00b557ce9b4d7ec288b67a4b24fc311cc4bd278d
Reviewed-on: https://chromium-review.googlesource.com/1069827
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py