Move get_server_or_ip method to common class

Moved to common class duplicate part of code for "get server or IP"
which used in few scenario tests.

Change-Id: I6368c24764e2515dc8bbcd82d1b0623c24bc77cd
diff --git a/tempest/scenario/test_shelve_instance.py b/tempest/scenario/test_shelve_instance.py
index 3b38888..0e366e9 100644
--- a/tempest/scenario/test_shelve_instance.py
+++ b/tempest/scenario/test_shelve_instance.py
@@ -78,24 +78,17 @@
             server = self.create_server(image=CONF.compute.image_ref,
                                         create_kwargs=create_kwargs)
 
-        if CONF.compute.use_floatingip_for_ssh:
-            floating_ip = self.create_floating_ip(server)['ip']
-            timestamp = self.create_timestamp(
-                floating_ip, private_key=keypair['private_key'])
-        else:
-            timestamp = self.create_timestamp(
-                server, private_key=keypair['private_key'])
+        instance_ip = self.get_server_or_ip(server)
+        timestamp = self.create_timestamp(instance_ip,
+                                          private_key=keypair['private_key'])
 
         # Prevent bug #1257594 from coming back
         # Unshelve used to boot the instance with the original image, not
         # with the instance snapshot
         self._shelve_then_unshelve_server(server)
-        if CONF.compute.use_floatingip_for_ssh:
-            timestamp2 = self.get_timestamp(floating_ip,
-                                            private_key=keypair['private_key'])
-        else:
-            timestamp2 = self.get_timestamp(server,
-                                            private_key=keypair['private_key'])
+
+        timestamp2 = self.get_timestamp(instance_ip,
+                                        private_key=keypair['private_key'])
         self.assertEqual(timestamp, timestamp2)
 
     @test.idempotent_id('1164e700-0af0-4a4c-8792-35909a88743c')