Since the Applicable code logs as it is being used as a decorator as
other files are being parsed, we need a logger before the command-line
options are parsed.  And now, since multiple source files might setup
the root logger, we ensure it only gets hooked up to the console once.
Subsequent calls to configure_logging only affect the logging level.

If you need early debug logging, set ELASTICSLICE_DEBUG to anything.

Oh, and I changed the default log level to WARN, from INFO.