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/base/init.sls

@@ -1,3 +1,8 @@
+base:
+  pkgrepo.managed:
+    - name: deb http://httpredir.debian.org/debian jessie-backports main
+    - file: /etc/apt/sources.list.d/backports.list
+
 base-packages:
   pkg.installed:
     - pkgs: