buffet: Send modelManifestId with device registration

modelManifestId is ID of manifest with useful information
that can be used by device clients.
modelManifestId is the same value as modelId from /privet/info.

CQ-DEPEND=CL:251381
BUG=brillo:295
TEST=unittests

Change-Id: Ic847c30badd9aa8b97f3deb7939813cc95195f02
Reviewed-on: https://chromium-review.googlesource.com/251141
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/device_registration_info.h b/buffet/device_registration_info.h
index 7046718..780aeb3 100644
--- a/buffet/device_registration_info.h
+++ b/buffet/device_registration_info.h
@@ -211,6 +211,7 @@
   std::string display_name_;
   std::string description_;
   std::string location_;
+  std::string model_id_;
 
   // Transient data
   std::string access_token_;