volume mount protopuf updates

> - stop using deprecated required field modifier
> - switch json tag for shared field to shared so that it matches the variable name
> [#130471951](https://www.pivotaltracker.com/story/show/130471951)

Author: Julian Hjortshoj

models/volume_mount.pb.go

@@ -12,7 +12,7 @@ enum DeprecatedBindMountMode {
 }
 
 message SharedDevice {
-  required string volume_id = 1 [(gogoproto.jsontag) = "volume_id"];
+  optional string volume_id = 1 [(gogoproto.jsontag) = "volume_id"];
   optional string mount_config = 2 [(gogoproto.jsontag) = "mount_config"];
 }
 
@@ -26,7 +26,7 @@ message VolumeMount {
   optional string mode = 6 [(gogoproto.jsontag) = "mode"];
 
   // oneof device {
-    optional SharedDevice shared = 7 [(gogoproto.jsontag) = "shared_device"];
+    optional SharedDevice shared = 7 [(gogoproto.jsontag) = "shared"];
   // }
 }