Move assign_keypair method in base class

Change-Id: I69b8db037991f88b38b293560532bf51c0a04378
diff --git a/scenario/test_volumes.py b/scenario/test_volumes.py
index 6a80297..a60ffb0 100644
--- a/scenario/test_volumes.py
+++ b/scenario/test_volumes.py
@@ -27,11 +27,7 @@
     def setUp(self):
         super(VolumeBackupRestoreIntegrationTest, self).setUp()
         self.client = self.orchestration_client
-        if self.conf.keypair_name:
-            self.keypair_name = self.conf.keypair_name
-        else:
-            self.keypair = self.create_keypair()
-            self.keypair_name = self.keypair.id
+        self.assign_keypair()
         self.volume_description = 'A test volume description 123'
         self.volume_size = self.conf.volume_size