Add yaml files for mongo persistent volume claim

gcloud/mongo-pv/mongo-pvc.yaml

+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: mongo-volume
+spec:
+  storageClassName: default-mongo
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 1Gi

gcloud/mongo-pv/mongo-storage-class.yaml

+apiVersion: storage.k8s.io/v1
+kind: StorageClass
+metadata:
+  name: default-mongo
+provisioner: kubernetes.io/gce-pd
+volumeBindingMode: Immediate
+allowVolumeExpansion: true
+reclaimPolicy: Delete
+parameters:
+  type: pd-standard
+  fstype: ext4
+  replication-type: none