Export template's relative path to ansible_managed

Given that `{file}` in _ansible_managed_ gets substituted to an absolute path, it can change in situations where an username is part of that path no matter if the template is the same file with respect to the playbook using it. For example _/home/foo/ansible-site/path/to/template.j2_ vs. _/home/bar/work/tests/ansible-site/path/to/template.j2_)

This changeset exports the template's relative path as `template_relpath` var and also substitutes `relfile` in _ansible_managed_.