Fix unit test to use python3 command

unit test jobs staretd to run on ubuntu-focal now
and failing for using 'python' command.

Change-Id: Ie002faf4c96ac7f207207a481c057b8df0289e6c
diff --git a/tests/test_write_devstack_local_conf_role.sh b/tests/test_write_devstack_local_conf_role.sh
index b2bc0a2..71d8d51 100755
--- a/tests/test_write_devstack_local_conf_role.sh
+++ b/tests/test_write_devstack_local_conf_role.sh
@@ -6,4 +6,4 @@
 source $TOP/functions
 source $TOP/tests/unittest.sh
 
-python ./roles/write-devstack-local-conf/library/test.py
+${PYTHON} $TOP/roles/write-devstack-local-conf/library/test.py