Change LOG.warn to LOG.warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning
so we prefer to use warning to avoid DeprecationWarning.

Change-Id: I3a057080b7b888b2c246ad9910ddba564b07dbd9
Closes-Bug: #1530742
diff --git a/tempest/common/fixed_network.py b/tempest/common/fixed_network.py
index 56cd331..3fc1365 100644
--- a/tempest/common/fixed_network.py
+++ b/tempest/common/fixed_network.py
@@ -49,13 +49,13 @@
                    name, networks))
         if caller:
             msg = '(%s) %s' % (caller, msg)
-        LOG.warn(msg)
+        LOG.warning(msg)
         raise exceptions.InvalidTestResource(type='network', name=name)
     else:
         msg = "Network with name: %s not found" % name
         if caller:
             msg = '(%s) %s' % (caller, msg)
-        LOG.warn(msg)
+        LOG.warning(msg)
         raise exceptions.InvalidTestResource(type='network', name=name)
     # To be consistent between neutron and nova network always use name even
     # if label is used in the api response. If neither is present than then
@@ -65,7 +65,7 @@
         msg = "Network found from list doesn't contain a valid name or label"
         if caller:
             msg = '(%s) %s' % (caller, msg)
-        LOG.warn(msg)
+        LOG.warning(msg)
         raise exceptions.InvalidTestResource(type='network', name=name)
     network['name'] = name
     return network
@@ -122,6 +122,6 @@
         if 'id' in network.keys():
             params.update({"networks": [{'uuid': network['id']}]})
         else:
-            LOG.warn('The provided network dict: %s was invalid and did not '
-                     ' contain an id' % network)
+            LOG.warning('The provided network dict: %s was invalid and did '
+                        'not contain an id' % network)
     return params