Add possibility to specify Compose file version

This change allows to set the Compose file version in the model instead
of the hardcoded value.

Change-Id: I5f7fb3c0770a48fe6dada54ffb9f925ef6ebd078
diff --git a/tests/pillar/client_deploy.sls b/tests/pillar/client_deploy.sls
index 860531a..d6d56ea 100644
--- a/tests/pillar/client_deploy.sls
+++ b/tests/pillar/client_deploy.sls
@@ -6,6 +6,7 @@
         enabled: true
         environment:
           SOMEVAR: somevalue
+        version: 3.3
         service:
           db:
             image: postgres