Avoid .sh extension for Jinja template

Summary:
Linter currently tests .sh files with shellcheck.

It then complains templates blocks starting with { are syntax error.

Templates should so use an extension to avoid to confuse linter,
and perhaps create a test task to render the template before to call shellcheck.

Test Plan: arc lint on D957

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D958