Generate junit per rspec thread

Multiple threads of rspec were writing into the same rspec.xml file. Gitlab was then unable to parse this file.

Note:

• We have to put rspec_parallel erb in the configuration file since its evaluated by rspec itself. Consequently we cannot pass it in via SPEC_OPTS in CI only. The downside is that is when running rake parallel_rspec locally we generate the junit xml files as well but that is not such a problem.
• <%% is for nested erb statements.

Thanks to @pigueira for pointing this fix out.

A test of this change (and the other things currently in HEAD) is available here - ai/it-puppet-module-bagplus!642 I think there are still problems passing some of the results but this is much better than it was before.

Second commit generates an rspec.xml for the rubocop results - the file is lacking in actual results - only contains a list of tests - worry about that later - it is still progress.