use a single dataplane instance for all tests

Creating and destroying dataplane instances was taking about 1/3 of the total
runtime.
diff --git a/src/python/oftest/__init__.py b/src/python/oftest/__init__.py
index c1cbb78..3974e08 100644
--- a/src/python/oftest/__init__.py
+++ b/src/python/oftest/__init__.py
@@ -3,3 +3,7 @@
 # Global config dictionary
 # Populated by oft.
 config = {}
+
+# Global DataPlane instance used by all tests.
+# Populated by oft.
+dataplane_instance = None