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
Merge pull request #703 from wking/701-consolidate-config-log

arbiter: Consolidate configuration logging