Allow loading other auth methods in auth_token

Add handlers for loading user specified authentication plugins from the
config file. If an auth plugin is not specified then we fall back to
using the older legacy options.

Change-Id: I4d09a30c0163106dab52062fab6000aaec0efb5d
Closes-Bug: #1372142
Blueprint: pluggable-auth
Merge "Allow loading other auth methods in auth_token"