SECRETS: Add DEBUG messages to the sssd-secrets provider
    
    Previously, it was not possible to follow the flow of the secrets
    responder or find out what went wrong on error. This patch adds DEBUG
    messages so that most failure cases have their own message. At the same
    time, running sssd-secrets with debug_level <= 3 does not emit any
    messages at all.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>