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/webserver/sslcert.macros.sls

@@ -9,6 +9,7 @@
     - bits: {{ salt['pillar.get']('nginx:keylength:' + domain_name, 2048) }}
     - require:
       - file: {{ nginx_ssl_keydir }}
+      - pkg: python-cryptography
     - require_in:
       - file: /etc/nginx/sites-available/{{ domain_name }}
       - service: nginx