Remove warning when rendering non-SLS files

[amendlik]

What does this PR do?

My Salt states contain code to render Jinja in YAML files using the slsutil.renderer function, such as:

{% set defaults = salt["slsutil.renderer"]("salt://defaults.yaml", sls="defaults.yaml") %}

This works fine, but it always produces the error message: "Failed to determine proper template path"

What issues does this PR fix or reference?

None

Previous Behavior

Running slsutil.renderer on any file other than an SLS file would work, but produce a superfluous warning.

New Behavior

Running slsutil.renderer on non-SLS files succeeds without a warning, and populates the following context variables:

• tplpath
• tplfile
• tpldir

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes