Fix st2 packs actions

This PR addresses problems around immutability and supplying default value for params. 

If a runner param is declared immutable in runner, the default value should be supplied in runner.
If an action param is declared immutable in action, default value should be supplied in action.
If a runner param is declared immutable in action, default value can either be the one from runner or if it's supplied in action, we'll pick that up.