buffet: Add D-Bus API to change GCD service settings

Added function UpdateServiceConfig that can be used to switch servers
for testing. Function modifies client_id, client_secret, api_key,
oauth_url and service_url.

BUG=brillo:1058
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: Ia8f8ba3ad57372ecedbff2819ce1991ba391a41d
Reviewed-on: https://chromium-review.googlesource.com/271387
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/manager.h b/buffet/manager.h
index 3961b68..479ac9b 100644
--- a/buffet/manager.h
+++ b/buffet/manager.h
@@ -67,6 +67,12 @@
                         const std::string& in_name,
                         const std::string& in_description,
                         const std::string& in_location) override;
+  bool UpdateServiceConfig(chromeos::ErrorPtr* error,
+                           const std::string& client_id,
+                           const std::string& client_secret,
+                           const std::string& api_key,
+                           const std::string& oauth_url,
+                           const std::string& service_url) override;
   void UpdateState(DBusMethodResponse<> response,
                    const chromeos::VariantDictionary& property_set) override;
   bool GetState(chromeos::ErrorPtr* error, std::string* state) override;