Explicitly remove config parameters repo

This was working, but because the Jenkins job had 'Delete workspace before build starts' checked. Better to be explicit about set-up in code rather than relying on options in Jenkins.

Also, no need to delete `Gemfile.lock` as this is covered by `git clean -x`. (Ref https://github.com/alphagov/vcloud-core/commit/996c140c131b).
Merge pull request #32 from alphagov/fixup-jenkins

Explicitly remove config parameters repo