Set default log level of iso8601 to WARN

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
Merge "Set default log level of iso8601 to WARN"