Add David Chadwick as ATC

Professor David Chadwick has been deeply involved in keystone's
[painstakingly slow] shift towards a federated identity model for over a
year now. I think all of keystone-core can vouch for the value of his
technical contributions on the matter. He regularly participates on the
-dev mailing list and in gerrit:

  https://review.openstack.org/#/q/owner:6534+OR+reviewer:6534,n,z

Change-Id: Ia2557c17c43fb280c85122d75e513472a320cc4d
Merge "Add David Chadwick as ATC"