.gitignore: add GCP files to .gitignore

Change-Id: Ic5819d4dc2af207c0fcad9d038095adad71a0770
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/447918
Reviewed-by: Robert Findley <rfindley@google.com>

.gitignore

@@ -43,4 +43,10 @@ override.tf.json
 ### Discovery Project ###
 sos.googlesource.com/sos
 *.zip
+
+#### GKE
 go-discovery*.json
+
+#### App Engine
+*credentials.yaml
+*.gcloudignore