buffet: Add the ability to change command visibility

Now it is possible to change the visibility of command and make it
visible to cloud only, local only, both or none.

BUG=brillo:797
TEST=`FEATURES=test emerge-link buffet`

Change-Id: I81d526b3d43adf5d6cd03a4e31a31e1494ff5c1b
Reviewed-on: https://chromium-review.googlesource.com/266396
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/commands/command_dictionary.h b/buffet/commands/command_dictionary.h
index 52e1a10..7668fba 100644
--- a/buffet/commands/command_dictionary.h
+++ b/buffet/commands/command_dictionary.h
@@ -78,6 +78,7 @@
   void Clear();
   // Finds a definition for the given command.
   const CommandDefinition* FindCommand(const std::string& command_name) const;
+  CommandDefinition* FindCommand(const std::string& command_name);
 
  private:
   using CommandMap = std::map<std::string, std::unique_ptr<CommandDefinition>>;