Added Keystone and RequestID headers to CORS middleware

Added Keystone and RequestID headers to CORS middleware

CORS middleware's latent configuration feature, new in 3.0.0,
allows adding headers that apply to all valid origins.
This patch adds headers commonly used in openstack to cinder's paste
pipeline, so that operators do not have to be aware of additional
configuration magic to ensure that browsers can talk to the API.

For more information:
http://docs.openstack.org/developer/oslo.middleware/cors.html#configuration-for-pastedeploy

Change-Id: I77980aad268fecd1d14cb57e6140f039c01d0eb4