Move assign_keypair method in base class

Change-Id: I69b8db037991f88b38b293560532bf51c0a04378
diff --git a/common/test.py b/common/test.py
index 7b17d57..bfb1260 100644
--- a/common/test.py
+++ b/common/test.py
@@ -200,6 +200,14 @@
         self.addCleanup(delete_keypair)
         return keypair
 
+    def assign_keypair(self):
+        if self.conf.keypair_name:
+            self.keypair = None
+            self.keypair_name = self.conf.keypair_name
+        else:
+            self.keypair = self.create_keypair()
+            self.keypair_name = self.keypair.id
+
     @classmethod
     def _stack_rand_name(cls):
         return rand_name(cls.__name__)