Fix AttributeError with run_validation=true

This fixes a regression in the recently merged change. [1]

[1] Ie7814bd50f1c57c5f8da8355f665f09c9ac8e2d9

Closes-Bug: #1595823
Change-Id: I135a52f052ed9acebff8e5c2f523a1b08a6fe01e
diff --git a/tempest/scenario/test_server_basic_ops.py b/tempest/scenario/test_server_basic_ops.py
index 91669d0..80728dc 100644
--- a/tempest/scenario/test_server_basic_ops.py
+++ b/tempest/scenario/test_server_basic_ops.py
@@ -56,7 +56,7 @@
             # Check ssh
             self.ssh_client = self.get_remote_client(
                 ip_address=self.fip,
-                username=self.image_utils.ssh_user(self.image_ref),
+                username=self.ssh_user,
                 private_key=keypair['private_key'])
 
     def verify_metadata(self):