Setup backports and ensure python-cryptography

Make sure that the jessie-backports repository is available and that the
python-cryptography package from that repository is used.

states/gnuviechadmin/webinterface.sls

@@ -12,8 +12,9 @@ gnuviechadmin-dev-packages:
       - pkg: gnuviechadmin-packages
 
 python-cryptography:
-  pkg.installed:
+  pkg.latest:
     - reload_modules: true
+    - fromrepo: jessie-backports
 
 {% import "webserver/sslcert.macros.sls" as sslcert %}