Run a default hook if the specific hook doesn't exist.

We now support a default hook called (oddly enough) "default-hook".  If a specific hook is missing and the default hook exists, the default hook will be called with the name of the hook as the first argument.

This has been tested live with a tweaked [postgres charm](https://github.com/natefinch/pgcharm).  This charm was tested because it was the first one I found that was all symlinks to a single python script.  Removed all symlinks and made a very minor tweak to the python script to grab the first argument and use it as the hook name.  It works... dare I say... like a charm.
(in particular, we could make sure we *always* inject that var into the context, rather than having two mechanisms for discovering hook name that work in different ways at different times)