Fix migration tests

Now we have part of skipped migration tests by reason of we have't parameter
"min_compute_nodes" in tempest.conf.

Change-Id: I14aa51f19221c662f8ea8712f2da4b4fdc2a19c1
Related-prod:PROD-24630
diff --git a/_modules/runtest/tempest_sections/compute.py b/_modules/runtest/tempest_sections/compute.py
index 6ae4439..783f997 100644
--- a/_modules/runtest/tempest_sections/compute.py
+++ b/_modules/runtest/tempest_sections/compute.py
@@ -162,7 +162,10 @@
 
     @property
     def min_compute_nodes(self):
-        pass
+        c = conditions.BaseRule(field='nova.compute.enabled', op='eq',
+                                val=True)
+        nodes = self.get_nodes_where_condition_match(c)
+        return len(nodes)
 
     @property
     def min_microversion(self):