Charm now validates YAML-loaded Actions schema

At the time the Charm's Actions schema is loaded from YAML, gojsonschema
now uses its Validate method to ensure that the loaded schema conforms to
JSON-Schema Draft 4.  The schema for JSON-Schema Draft 4 has been added
as a .json file to the charm package in order to avoid remote queries to the spec.
Also consider whether we actually want to export this constant.