Accessing action_context directly within a python-runner

Often when carrying out actions to external services you want to act as the requesting user (e.g. creating releases, deployments or issues on GitHub) however the only way to get the api user running the alias into the action is as by adding a parameter to the action as follows (the default is to prevent it failing when run via the ST2 WebUI):

  api_user:
    type: "string"
    description: "The API user"
    default: "{{action_context.api_user|default(None)}}"

It would be so much easier if action_context was available in Python, then you could not mess around with adding parameters and just access them directly as follows:

if action_context.api_user:
  do things as API user... 
else:
  do things as user....

And this would keep the action alot cleaner and prevent issues happening due to api_user not being set.

Trace how the Python runner invokes actions and how action context is currently passed into them. Define the supported direct access to action_context, including behavior when api_user is absent, and add coverage showing that Python actions can distinguish API-user and user execution contexts.