slapi-nis: don't search in SSSD when memberUid has no '@' separator
    
    In the case there are no groups in cn=groups map that have certain
    memberUid as a member, we look at possibility that this user might
    be coming from a trusted AD forest. However, all users from trusted
    AD forests do have '@' separator in the name between the user name
    and the domain.
    
    In case there is no '@' separator, consider such search as not valid
    for lookups in SSSD.
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1243823