Force catch_errors into pipeline

This commit adds a hook for WSGI applications
(e.g. proxy.server.Application) to modify their WSGI pipelines. This
is currently used by the proxy server to ensure that catch_errors is
present; if it is missing, it is inserted as the first middleware in
the pipeline.

This lets us write new, mandatory middlewares for Swift without
breaking existing deployments on upgrade.

Change-Id: Ibed0f2edb6f80c25be182b3d4544e6a67c5050ad
Merge "Force catch_errors into pipeline"