update google-auth to 1.15.0 part 5 [(#3818)](GoogleCloudPlatform/python-docs-samples#3818)

Author: leahecole

samples/api-client/manager/requirements.txt

@@ -2,7 +2,7 @@ cryptography==2.9.2
 flaky==3.6.1
 google-api-python-client==1.8.2
 google-auth-httplib2==0.0.3
-google-auth==1.14.3
+google-auth==1.15.0
 google-cloud-iot==1.0.0
 google-cloud-pubsub==1.4.3
 paho-mqtt==1.5.0