Bump default nginx version from 1.13 to 1.15 (#4429)
Bump default nginx version from 1.13 to 1.15 (#4429)


RSulzmann/openwhisk,sven-lange-last/openwhisk,rabbah/openwhisk,openwhisk/openwhisk,RSulzmann/openwhisk,akrabat/openwhisk,jeremiaswerner/openwhisk,style95/openwhisk,sven-lange-last/openwhisk,sven-lange-last/openwhisk,csantanapr/incubator-openwhisk,sven-lange-last/openwhisk,akrabat/openwhisk,jeremiaswerner/openwhisk,RSulzmann/openwhisk,csantanapr/incubator-openwhisk,rabbah/openwhisk,jeremiaswerner/openwhisk,houshengbo/openwhisk,markusthoemmes/openwhisk,RSulzmann/openwhisk,openwhisk/openwhisk,csantanapr/incubator-openwhisk,jasonpet/openwhisk,style95/openwhisk,jasonpet/openwhisk,jeremiaswerner/openwhisk,markusthoemmes/openwhisk,csantanapr/incubator-openwhisk,akrabat/openwhisk,csantanapr/incubator-openwhisk,sven-lange-last/openwhisk,markusthoemmes/openwhisk,houshengbo/openwhisk,houshengbo/openwhisk,jeremiaswerner/openwhisk,markusthoemmes/openwhisk,markusthoemmes/openwhisk,style95/openwhisk,style95/openwhisk,rabbah/openwhisk,akrabat/openwhisk,csantanapr/incubator-openwhisk,jeremiaswerner/openwhisk,houshengbo/openwhisk,akrabat/openwhisk,csantanapr/incubator-openwhisk,jeremiaswerner/openwhisk,markusthoemmes/openwhisk,RSulzmann/openwhisk,rabbah/openwhisk,jasonpet/openwhisk,sven-lange-last/openwhisk,jasonpet/openwhisk,RSulzmann/openwhisk,markusthoemmes/openwhisk,RSulzmann/openwhisk,jasonpet/openwhisk,houshengbo/openwhisk,jasonpet/openwhisk,openwhisk/openwhisk,rabbah/openwhisk,openwhisk/openwhisk,openwhisk/openwhisk,openwhisk/openwhisk,sven-lange-last/openwhisk,markusthoemmes/openwhisk,houshengbo/openwhisk,jasonpet/openwhisk,style95/openwhisk,akrabat/openwhisk,houshengbo/openwhisk,style95/openwhisk,style95/openwhisk,akrabat/openwhisk,rabbah/openwhisk,rabbah/openwhisk