Allow ETL files without a DOCTYPE without validation warnings

[ejboy]

Expected behavior:

• ETL files without a DOCTYPE execute without XML “no grammar found” warnings.
• Files declaring etl.dtd continue using bundled DTD validation.
• Add tests for both cases.
• Preserve the existing validation configuration API.

The likely fix belongs in ConfigurationFactory: enable DTD validation only when a DOCTYPE is present, rather than enabling parser validation globally.