Provide Salt module to convert data

Summary:
salt['convert.to_json_from_pillar_key']('foo') returrns a JSON
representation of the pillar key foo.

The goal is to allow to configure a software in YAML, but to output
the configuration as a JSON file, format more suitable for data
interchange.

Test Plan: Tested with D1681.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: http://devcentral.nasqueron.org/D1684