Mixed authentication modes

Configure multiple authentication methods in order. This allows users to specify the preferred method of authentication, but provide a fallback if that fails.

We would rather not create service users in ldap for the purpose of interacting with collins, and this allows us to do without affecting existing users and perhaps providing flexibility for future adopters.

One note regarding the tests --
This test passes individually (i.e. ```test-only util.security.MixedAuthenticationProviderSpec```), but fails as part of the global ```play test```. This is because configurations from previous tests bleed over between tests. At some point the ```authentication_reference.conf``` file is being loaded and polluting the configurations for ldap, which makes the ldap provider actually attempt a lookup and fail.