Make stackenv file visible

The file won't be listed as long as it is called .stackenv.txt.gz, with
this it will be called _stackenv.txt.gz instead.

Change-Id: Ib3b44c287ffb2ec0e48fefef1662a1c02d162657
diff --git a/playbooks/post.yaml b/playbooks/post.yaml
index 36023a6..a2f695c 100644
--- a/playbooks/post.yaml
+++ b/playbooks/post.yaml
@@ -45,6 +45,7 @@
         - conf
         - log
         - localrc
+        - stackenv
         - summary
     # NOTE(andreaf) We need fetch-devstack-log-dir only as long as the base job
     # starts pulling logs for us from {{ ansible_user_dir }}/logs.