Ensure st2.conf values are not converted to particular structure

[arm4b]

Closes #238

Covers edge case when user wants to set in st2.conf json-formatted value which might be converted to particular structure during the jinja/ansible pipeline.

See #156

[Kami]

LGTM.

How hard would it be to add tests for it?

[arm4b]

That would be not that easy and bring insufficient value comparing to increasing the build timing and tests complexity.
Ansible plays we have are designed to be simple and have only smoke tests instead of checking all the permutations of possible configurations. It's the best effort we can do and is good enough, considering ongoing development/maintenance of many other projects.