sync-devstack-data: new argument devstack_data_base_dir

When the role is used by grenade, the data directory is shared among
different devstack executions, and the base directory is different,
for example: /opt/stack/data vs /opt/stack/{old,new}.
The new devstack_data_base_dir parameter allows user to specify
a base directory for the data/ directory which is unrelated
to the devstack directory. The default value is devstack_base_dir,
so the default behavior is unchanged.

Change-Id: Ie69b7b51947cbf1a8b31d2701783de2fb56a2d33
diff --git a/roles/sync-devstack-data/README.rst b/roles/sync-devstack-data/README.rst
index 500e8cc..388625c 100644
--- a/roles/sync-devstack-data/README.rst
+++ b/roles/sync-devstack-data/README.rst
@@ -10,3 +10,10 @@
    :default: /opt/stack
 
    The devstack base directory.
+
+.. zuul:rolevar:: devstack_data_base_dir
+   :default: {{ devstack_base_dir }}
+
+   The devstack base directory for data/.
+   Useful for example when multiple executions of devstack (i.e. grenade)
+   share the same data directory.