Privatize auth construction parameters

Privatize some of the automatic auth plugin construction work. At some
point we are going to need to have this pluggable and i'm not sure the
current methods will suffice. It's better to keep this private until we
are sure rather than be stuck with a public API.

Change-Id: I2a10a9b28bef6c094b1330a0524f1c516f5103fd
Closes-Bug: #1287488
Merge "Privatize auth construction parameters"