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
remove audit logging on flush

in something as simple as the tempest gate, this creates 2.5 MB
of compressed logs. I do not believe there is any actual value
in logging the flush, especially as the failure to flush is
correctly logged.

Change-Id: I176bc13e5cacf892b07c4fed3871924500622d01
Merge "remove audit logging on flush"
Merge "Set default log level of iso8601 to WARN"