OSGi/catalog improvements

It feels confusing when code like this, which is using the Catalog API, should use methods on `CatalogDto` versus on the catalog directly. For example, the `BasicBrooklynCatalog.toXmlString` is how I'd generate the XML, but I'd need to call `CatalogDto.newDtoFromXmlContents` to convert the XML back to a DTO (which is in the internal package name).