tools (common/template): add filter to get media types: Add 2 filters to get main type and subtype of media type. Jinja2 and Nunjucks don't handle slices in the same way (Python way for Jinja2, JS way for Nunjucks), making it difficult to retrieve main type of a media from media type. Thoses filters work in both cases.

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)