arbiter: Consolidate configuration logging

Benoit prefers the log prefix for the first line ("Current configuration:")
but not subsequent lines (" {config}: {value}") [1], so consolidate to a
single log.debug call.  The newer '{0}'.format() syntax requires Python 2.6
[2], but our setup.py only claims compatibility with 2.6, 2.7, 3.2, and 3.3,
so that should be fine.

[1]: https://github.com/benoitc/gunicorn/pull/701/files#r9767234
[2]:
http://docs.python.org/2/whatsnew/2.6.html#pep-3101-advanced-string-formatting