Don't monkeypatch pecan.core.Request in newer versions of pecan

An upcoming version of pecan will allow you to specify a custom pecan.request
implementation in the Pecan WSGI application constructor.  Detect support for
this functionality and attempt to utilize it instead of the current
monkeypatching approach.

Conflicts:
	designate/api/v2/app.py

Change-Id: I81008ab2fd2390994db640db0c781ee20f5b33a1