Fix periodic CI jobs

Periodic jobs don't have zuul.change defined, since there is no change
being tested. This causes an early failure when referencing zuul.change
to set the image tag for built images. In periodic jobs we'll never need
to build images because there is no dependent kolla change under test.

Change-Id: I6d9d81cf17b7d0d7aaf87cd96418c904c46681f2