Replace assertions with more specific ones

This patch adds the already implemented assert(Not)Empty functions
instead of generic assert functions where assert(Not)Equal or
assertGreater(Equal) is used to check the state of a container.
This change makes the code and the error messages more readable,
therefore it improves maintainability a bit.

Change-Id: Ib7df910c7853141bcf2effd89f8fba864fc6bbb5
diff --git a/tempest/api/compute/admin/test_networks.py b/tempest/api/compute/admin/test_networks.py
index 0ea0a78..acb0d90 100644
--- a/tempest/api/compute/admin/test_networks.py
+++ b/tempest/api/compute/admin/test_networks.py
@@ -62,4 +62,4 @@
             self.assertIn(configured_network, [x['label'] for x in networks])
         else:
             network_labels = [x['label'] for x in networks]
-            self.assertGreaterEqual(len(network_labels), 1)
+            self.assertNotEmpty(network_labels)