Prepare for future i18n use of _() in nova notifier.

Use the correct verion of _() function in nova notifier.

This is part of the blueprint gettext-i18n-issue.

Change-Id: I5204d7268c4c6124c7aef9c88cd4efacf7d00b5b
Merge "Prepare for future i18n use of _() in nova notifier."