YAQL functions API

We already implemented the possibility to plug in new YAQL functions (see https://blueprints.launchpad.net/mistral/+spec/mistral-custom-yaql-functions and related patches), the next step is to establish API that YAQL functions can safely use. That API should be considered public and should be maintained in backwards compatible manner.

This API should primarily include functions to query persistent Mistral entities like workflow/task/action executions, cron/event triggers etc. It can also potentially provide functions to access some configuration options (in read-only mode) and more convenient access to workflow context.