IFP: Fix lookups with fully-qualified names
    
    The Data Provider lookup code user the original input string as the
    lookup key instead of the parsed name component. For example, for an
    input joe@mydomain, the backend would have searched for:
        (&(cn=joe@mydomain)(objectclass=user))
    
    This patch fixes the lookup to use the parsed name.
    
    https://fedorahosted.org/sssd/ticket/2402
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>