Added rule: check that jinja template files have the correct extension

[jbouter]

When using file.recurse, one might have jinja templates in there that can't be renamed to have the .j2 extension, because then they'll have that extension on filesystem.

For example:

nginx_vhosts:
  file.recurse:
    - name: /etc/nginx/sites-enabled/
    - source: salt://nginx/vhosts
    - template: jinja
    [..]

If /nginx/vhosts/ is filled with vhost.conf.j2 files, they will be put on the server's filesystem as vhost.conf.j2 files.

[jbouter]

First of all, thank you for your contribution!

We are, however, sadly closing this pull request due to the following reasons:

• We should only be linting SaltStack files, meaning only lint files ending in .sls. We don't want our users to lint all of their files, scanning for possible jinja.
• We can't rule out false positives as stated in the comment above.

I suggest this PR is rewritten to do the following:

• Check only .sls files
• Check if file.managed is being used
• If so, check if - template: jinja is set
• If these conditions match, check if a - source: is set
• Check if the file path in - source: ends with .j2

If you do wish however to use this code, you can use -r option to specify your own set of rules in your own SaltStack linting pipeline. But we won't be merging this into the codebase.