Delete before creating new snaps (#84)

according to the docs https://cloud.google.com/compute/docs/disks/restore-and-delete-snapshots#deleting_snapshot this would be slightly more correct since instead of gcp handling the magic of references, we can allow the newly created snapshot to reference the right parent

gcloud-snapshot.sh

@@ -376,12 +376,12 @@ main()
 
         # build snapshot name
         local snapshot_name=$(createSnapshotName ${PREFIX} ${device_name} ${DATE_TIME})
+
+        # delete snapshots for this disk that were created older than DELETION_DATE
+        deleteSnapshots "$PREFIX-.*" "$DELETION_DATE" "${device_id}"
 
         # create the snapshot
         createSnapshot ${device_name} ${snapshot_name} ${device_zone}
-
-        # delete snapshots for this disk that were created older than DELETION_DATE
-        deleteSnapshots "$PREFIX-.*" "$DELETION_DATE" "${device_id}"
     done
 
     logTime "End of google-compute-snapshot"