Fix 2904: "dsreplication enable" cannot initialize b11 from b10 because the schema initialization fails on the config schema.
The code already tests whether the new objectclass is part of the config schema and then excludes it from the import. That's fine.
Except that the test relies on the decoding of the OC and fails if the new OC comes with a new attribute.
The fix consists in relaxing the decoding when doing the test .. and really doing a strict decoding after the test once we know the new OC is not a config OC.