Refactor '_get_default_network' method in the integration tests

Refactored '_get_default_network' to get any network by its name,
othewise it was impossible to get 'public' net.

Change-Id: Ie691f99961a28e47f7437f3e52be9baf2984e839
diff --git a/scenario/scenario_base.py b/scenario/scenario_base.py
index b08766a..77c3624 100644
--- a/scenario/scenario_base.py
+++ b/scenario/scenario_base.py
@@ -21,7 +21,10 @@
         self.client = self.orchestration_client
         self.sub_dir = 'templates'
         self.assign_keypair()
-        self.net = self._get_default_network()
+
+        if not self.conf.fixed_network_name:
+            raise self.skipException("No default network configured to test")
+        self.net = self._get_network()
 
         if not self.conf.image_ref:
             raise self.skipException("No image configured to test")