8. core: Use new authentication interfaces

This change modifies the engine to use the new authentication interfaces
introduced in a previous patch.
With this new interfaces the engine is
prepared to use new authenticators and directories developed using the
new interfaces.

Change-Id: I2d5d6ee41b5373328eaf2f8448efe8e598609651
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>