Fixed unformatted message string

In _check_server_connectivity() method of
tempest/scenario/test_network_basic_ops module,
message is not properly formatted.

Formatted the message string properly.

Closes-Bug: 1483557
Change-Id: I719106acdacf0c75a86c20eba9c62e27a3a99f0c
diff --git a/tempest/scenario/test_network_basic_ops.py b/tempest/scenario/test_network_basic_ops.py
index e676063..12af667 100644
--- a/tempest/scenario/test_network_basic_ops.py
+++ b/tempest/scenario/test_network_basic_ops.py
@@ -338,8 +338,8 @@
 
         for remote_ip in address_list:
             if should_connect:
-                msg = "Timed out waiting for "
-                "%s to become reachable" % remote_ip
+                msg = ("Timed out waiting for %s to become "
+                       "reachable") % remote_ip
             else:
                 msg = "ip address %s is reachable" % remote_ip
             try: