Fix changelog generation for non-default versions

Currently if you generate a changelog for r0.1.1 of an API, you'd get "No significant changes" which is wrong. You should get a real changelog for the version.

This is now handled by generating a "preferred" changelog which acts as the default for version variables in the RST. Using a specific version's changelog is still supported for the rare cases where that is desired.

scripts/templating/matrix_templates/sections.py

@@ -41,7 +41,7 @@ class MatrixSections(Sections):
                 version_var = "%s_%s" % (spec_var, version)
                 logger.info("Rendering changelog for %s" % version_var)
                 rendered[version_var] = changelog
-                if version == "unstable":
+                if version == "preferred":
                     rendered[spec_var] = changelog
         return rendered