Use keystone.wsgi.Request for RequestClass

Keystone defines a custom Request class that's used in many
places (middleware, etc.). BaseApplication says that subclasses
should typically set Request as the custom RequestClass,
but for some reason it's not used in keystone.wsgi.Application.

This doesn't cause any problems at this point because Keystone's
custom Request is the same as webob.Request. bp user-locale-api
requires the custom Request to implement calculating the requested
locale, and if Application doesn't set this custom RequestClass then
most requests don't using the custom Request.

Part of changes for bp user-locale-api

Change-Id: If20ee9000aba89a5a2c94ed8a3dda7382142038e
Merge "Use keystone.wsgi.Request for RequestClass"