use nightly tag to pull openwhisk core images (#450)

Author: dgrove-oss

tools/travis/test_openwhisk.sh

@@ -85,7 +85,7 @@ cd $OPENWHISK_HOME
 
 #  Fire up the cluster
 cd $OPENWHISK_HOME/ansible
-ANSIBLE_CMD="ansible-playbook -i environments/local -e docker_image_prefix=openwhisk"
+ANSIBLE_CMD="ansible-playbook -i environments/local -e docker_image_prefix=openwhisk -e docker_image_tag=nightly"
 $ANSIBLE_CMD setup.yml
 $ANSIBLE_CMD prereq.yml
 $ANSIBLE_CMD couchdb.yml