Wrap auth req & resp w/ container (bug 1131840)

- Auth requests are now wrapped in an `auth` container
- The `authentication` attribute in authenticaiton requests is renamed
  to `identity`, as this attribute is reserved for containing objects
  for identifying the API user
- Auth responses are now wrapped in a `token` container
- Auth errors may now include an `identity` object instead of having
  an `authentication` sibling

Change-Id: I0b0a71fc6185e95961d173a96b49efca24a87636