Finish factoring apart the graph_action module

Factor out the scheduling, running and completion components
of graph_action so that we can allow this to be plugged in with
other types of scheduling, running and completion strategies.

The newly added components are the following:

- A runtime container class (serves as a holder of some small
  utility functions) and all the other runtime components.
- A runner class that acts as the action engines run loop.
- A scheduler class that schedules nodes using a provided executor
  and returns futures that can be used to introspect there results as
  they complete.
- A completer class that completes nodes and futures that the
  scheduler started, persisting there results and doing any further
  post-execution analysis.

Part of blueprint plug-engine

Change-Id: I1dbf46654377fc34e9d90eeabf7b0062020bdc5e
Merge "Finish factoring apart the graph_action module"