privetd: Implement privet/v3/commands/execute

Privetd forwards JSON with command to buffet. Buffet updates command and
returns JSON representation of the command.

BUG=brillo:428
TEST=unittest

Change-Id: I7bdd4a70dd94e3dc3b4a974c7ac734e44564ac6e
Reviewed-on: https://chromium-review.googlesource.com/262216
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>

buffet/buffet_client.cc

diff --git a/buffet/buffet_client.cc b/buffet/buffet_client.cc
index 79e7cdf..355d39b 100644
--- a/buffet/buffet_client.cc
+++ b/buffet/buffet_client.cc
@@ -356,7 +356,8 @@
 
   void CallAddCommand(const std::string& command, ManagerProxy* manager_proxy) {
     ErrorPtr error;
-    if (!manager_proxy->AddCommand(command, &error)) {
+    std::string id;
+    if (!manager_proxy->AddCommand(command, &id, &error)) {
       return ReportError(error.get());
     }
     OnJobComplete();