ipasam: use base scope when fetching domain information about own domain
    
    Since we use associatedDomain attribute to store information about UPN suffixes
    and our own domain, searching subtree is going to return more than one entry.
    Limit search for own domain by base scope as we only need to fetch our own
    domain information here, not UPN suffixes.
    
    Required for https://fedorahosted.org/freeipa/ticket/2945