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/README.rst b/README.rst
index deeb54d..ef6b711 100644
--- a/README.rst
+++ b/README.rst
@@ -119,6 +119,7 @@
             update: true
             environment:
               SOMEVAR: somevalue
+            version: "3.1"
             service:
               db:
                 image: postgres
diff --git a/docker/files/docker-compose.yml b/docker/files/docker-compose.yml
index fbe8b74..f120327 100644
--- a/docker/files/docker-compose.yml
+++ b/docker/files/docker-compose.yml
@@ -1,4 +1,4 @@
-version: '3'
+version: '{{ compose.version|default("3") }}'
 
 services:
   {%- for name, srv in service.iteritems() %}
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