res_pjsip_outbound_authenticator_digest: Add context to log messages

There was no context info in this module's log messages so it was
impossible to toubleshoot.

Added endpoint or host to all messages and added the realms in the
challenge for the "No auth credentials for any realm" message.

Change-Id: Ifeed2786f35fbea7d141237ae15625e472acff9b