Use stage_dir instead of /opt/stack

Historically we have collected devstack logs under /opt/stack.
Stop doing that and collect them in the stage_dir instead, so that
once the base job logs pull service comes around we are ready for it.

This add the benefit of writing things into a folder which is
already owned by the ansible user (ansible_user_dir), so we don't
run into issue writing there.

A few logs (devstack log, log summary and dstat) use to show up on
logs.o.o. just because they happened to already be in /opt/stack/logs.
With this change they would be lost, so adding them to post.yaml.

Depends-on: I5ad4dfccbc1389da3afc53f3c866d3475e006db6

Change-Id: Ib4be2f5056c0dc2b776de4a0d18b47b12624be92
diff --git a/roles/export-devstack-journal/README.rst b/roles/export-devstack-journal/README.rst
index 5f00592..a34e070 100644
--- a/roles/export-devstack-journal/README.rst
+++ b/roles/export-devstack-journal/README.rst
@@ -5,11 +5,17 @@
 kernal and sudo messages.
 
 Writes the output to the ``logs/`` subdirectory of
-``devstack_base_dir``.
+``stage_dir``.
 
 **Role Variables**
 
 .. zuul:rolevar:: devstack_base_dir
    :default: /opt/stack
 
-   The devstack base directory.
+   The devstack base directory. This is used to obtain the
+   ``log-start-timestamp.txt``, used to filter the systemd journal.
+
+.. zuul:rolevar:: stage_dir
+   :default: {{ ansible_user_dir }}
+
+   The base stage directory.