In openstack.common.log, the iso8601's default log level is WARN.
But ceilometer doesn't set WARN as default log level of iso8601.
This may cause a lot of DEBUG level message recorded in log files if we
haven't configured the default log level in ceilometer.conf.
Change-Id: I8edbcd5f0885b424c70483180b479588bca254a2
Closes-Bug: #1284412