python: Use getattr() and setattr() instead of __dict__.

python: Use getattr() and setattr() instead of __dict__.

This leaves one use of __dict__ used for iterating through attributes.
I could use dir() instead, but I was put off by this note in its
documentation in the Python Library Reference:

    Because dir() is supplied primarily as a convenience for use at an
    interactive prompt, it tries to supply an interesting set of names more
    than it tries to supply a rigorously or consistently defined set of names,
    and its detailed behavior may change across releases.  For example,
    metaclass attributes are not in the result list when the argument is a
    class.

Suggested-by: Reid Price <reid@nicira.com>