CustodiaClient: use ldapi when ldap_uri not specified
    
    Reinstate ldap_uri selection behaviour that was lost during the
    refactor in the previous commit.  This is required because, on the
    ipa-4-7 branch at least, the IPASecStore needs to use LDAPI to set
    the Directory Manager passphrase digest.  Use LDAPI when the
    ldap_uri has not been explicity specified.
    
    Part of: https://pagure.io/freeipa/issue/7964
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>