Add __repr__ handler for NovaObjects

This adds a __repr__ implementation for the base NovaObject which
should help debugging by compactly describing the contents of an
object. This adds stringification helpers to the Field object(s),
which lets us control what each type looks in compact format. Since
this is just for debugging, we avoid recursing too deeply on nested
objects and the like.

Change-Id: Ic604acfda7e7c7adb913479355726e16ad375f1d
Merge "Add __repr__ handler for NovaObjects"