The KNI jobs appear to be building the master branch even after
fetching proposed changes to gerrit. Change the choosing strategy
from 'default' to 'gerrit' to correct this.
See https://plugins.jenkins.io/gerrit-trigger/ for docs.
Change-Id: I028cde257415b2eb297cfa100f68d109df3fac71 Signed-off-by: James E. Blair <jeblair@redhat.com>