Fix mismatch error due to map in python3

When executed by python3 the test failed on a mismatch_error
caused by the fact that map function works differently in
python3.
The review fixes the issue by rewriting the line filtering
extensions to list comprehension.

Closes-Bug: #1829475

Change-Id: Ic2db882e505e7992192767acbe12a153b2d879e9
diff --git a/tempest/api/compute/test_extensions.py b/tempest/api/compute/test_extensions.py
index 34faf5f..12e7fea 100644
--- a/tempest/api/compute/test_extensions.py
+++ b/tempest/api/compute/test_extensions.py
@@ -37,7 +37,7 @@
         ext = CONF.compute_feature_enabled.api_extensions[0]
 
         # Log extensions list
-        extension_list = map(lambda x: x['alias'], extensions)
+        extension_list = [x['alias'] for x in extensions]
         LOG.debug("Nova extensions: %s", ','.join(extension_list))
 
         if ext == 'all':