Add Python-based hook scripting capability

```
An enhancement.

In the same way that external scripts are executed in response to the
state-change hooks, there is a need for the same capability to run Python code
at these points. The external scripts do not have access to the Python
environment, so they cannot make decisions based upon the current state of that
environment.

For example, setting an environment variable that contains the IP addresses of
all the configured nodes in the CONFIGURATION phase.

```

Original issue reported on code.google.com by `randomgr...@gmail.com` on 26 Jan 2012 at 6:14

The issue does not name implementation files or tests. Start by locating the state-change hook execution path and the CONFIGURATION phase, then determine how external scripts are invoked and where the current Python environment is available. Done means Python code can run at those hook points and access current state such as configured node IP addresses.

Written by the indexing model from the issue text.