String interpolation should be delayed to be handled by the logging code,
rather than being done at the point of the logging call.
Ref:http://docs.openstack.org/developer/oslo.i18n/guidelines.html#adding-variables-to-log-messages
For example:
WRONG: LOG.info(_LI('some message: variable=%s') % variable)
RIGHT: LOG.info(_LI('some message: variable=%s'), variable)
Change-Id: Icf027d87adc6d552711d178c54640d2f8b6af662