find_entry_by_attr() should fail if multiple entries are found

It will only ever return one entry so if more than one are found
then we raise an exception. This is most easily seen in the host
plugin where we search on the server shortname which can be the
same across sub-domains (e.g. foo.example.com &
foo.lab.example.com).

https://fedorahosted.org/freeipa/ticket/1388

ipaserver/plugins/ldap2.py

@@ -620,7 +620,12 @@ class ldap2(CrudBackend, Encoder):
         """
         search_kw = {attr: value, 'objectClass': object_class}
         filter = self.make_filter(search_kw, rules=self.MATCH_ALL)
-        return self.find_entries(filter, attrs_list, base_dn)[0][0]
+        (entries, truncated) = self.find_entries(filter, attrs_list, base_dn)
+
+        if len(entries) > 1:
+            raise errors.SingleMatchExpected(found=len(entries))
+        else:
+            return entries[0]
 
     def get_entry(self, dn, attrs_list=None, time_limit=None,
                   size_limit=None, normalize=True):