Use Python3 by default for devstack

Author: EmilienM

action.yaml

@@ -47,6 +47,7 @@ runs:
         SERVICE_PASSWORD=secret
         SWIFT_HASH=1234123412341234
         LOGFILE=${{ inputs.log_dir }}/devstack.log
+        USE_PYTHON3="True"
         EOF
         # horizon does not work in github action, permission error
         # dstat log takes too much memory to be stored by log artifacts