Add require-approval to Gerrit trigger

This feature allows Zuul to consider existing (or new) approval
votes associated with a change when determining whether an event
matches.  For example, it can be used to require that a Verified
vote of a certain age be present before a change is enqueued in
a pipeline.

Change-Id: I81344713d71b345b08576334568b9c49c810c7e9