Implement requesting device by index

For now main goal is to be able to request specific Vega card.
In the longer term we can use this to select compute device
which is a non-display, or the one which is on specific PCI
slot or so.

benchmark/foundation/context.py

@@ -8,6 +8,7 @@ class Context:
                  'device_type',
                  'device_name',
                  'device_single',
+                 'device_index',
                  'scenes',
                  'scenes_dir',
                  'configure_script',
@@ -23,6 +24,8 @@ class Context:
         self.device_name = 'NAME'
         # Use single device when there are multiple one matching the name
         self.device_single = False
+        # Use specified device index when device_single is truth.
+        self.device_index = -1
         # By default we use empty list, it is up to the user to fill it in.
         self.scenes = []
         # It is up to the user to provide proper path to scenes.