[SPT] Fixed the 'external_network' parameter when getting ext net

'external_network' worked not correclty since in case user sets it,
the method did not get it via API, so returned external_network
name instead of the dict with details of net.

Change-Id: Ia6e7a9d3b994399d04b06ab8b1ec47d1bc8c5153
Related: PROD-35300
diff --git a/test_set/cvp-spt/utils/os_client.py b/test_set/cvp-spt/utils/os_client.py
index f63afbf..2fd7237 100644
--- a/test_set/cvp-spt/utils/os_client.py
+++ b/test_set/cvp-spt/utils/os_client.py
@@ -270,10 +270,14 @@
                 if net["admin_state_up"] and net["router:external"] and
                 len(net["subnets"])
             ]
-            if networks:
-                ext_net = networks[0]
-            else:
-                ext_net = self.create_fake_external_network()
+        else:
+            networks = [net for net in self.os_clients.network.list_networks()["networks"]
+            if net["name"] == ext_net]
+ 
+        if networks:
+            ext_net = networks[0]
+        else:
+            ext_net = self.create_fake_external_network()
         return ext_net
 
     def create_flavor(self, name, ram=256, vcpus=1, disk=2):