Add functionality to versionHelper.py to invoke update-dtbook-metadata.jar
Add functionality to versionHelper.py to invoke update-dtbook-metadata.jar

basically by shelling out to a java process. This will allow us to
update the metadata without resorting to built-in buggy xsl
implementations (lxml). In the context of DOCTYPE entities lxml has a
few bugs which cause the content to become invalid under some
circumstances.

update-dtbook-metadata.jar should handle this correctly.

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