Fix French locale build
Fix French locale build

https://review.openstack.org/352007 created an issue with the French
locale, as it set an empty PO-Revision-Date: value. That made catalog
build fail with:

ValueError: time data u'' does not match format '%Y-%m-%d %H:%M'

Change-Id: I6f0c3622643a50701617ac031ac9cd3b7ed3649a
openstack/aodh,openstack/aodh