Add WWW-Authenticate header in 401 responses.

This review come from https://review.openstack.org/#/c/41707/, due to the
inactive in that.

Add the WWW-Authenticate in header when Keystone responds with 401 status
code. The format of the header is

WWW-Authenticate: Keystone uri="http://localhost:5000/"

as indicated in

http://docs.openstack.org/developer/keystone/middlewarearchitecture.html#exchanging-user-information .

Closes-Bug: #1153719
Change-Id: Iaa2291c66a9b126982b8aad0ddbfa9468dfc5d3e
Merge "Add WWW-Authenticate header in 401 responses."