zon.py: Use more pythonic string formatting.

The print statement was previously using string concatenation combined
with casting to the correct format. The str.format method automatically
casts as required.
zon.py: Simplify instance listing

Newer versions of boto provide the list_only_instances() method which is
a more direct way to get the instance listing.
Merge pull request #1 from dwijnand/lotia/python_cleanups

zon.py cleanups