Skip creating the formatted log message
if the message is not going to be emitted
because of the log level.
See the oslo i18n guideline.
* http://docs.openstack.org/developer/oslo.i18n/guidelines.html#\
adding-variables-to-log-messages
Change-Id: Ie9f3c9179cdae57ee298149f829811a5422fb9aa
Closes-Bug: #1596829