Pin python-cryptography to jessie-backports

Needed to satisfy leap-mx dependency (>=17.0)

- Resolves: #8837

puppet/modules/site_apt/manifests/preferences/python_cryptography.pp

+# Pin python-cryptography to jessie-backports in order to
+# satisfy leap-mx dependency (>=17.0)
+# see https://0xacab.org/leap/platform/issues/8837
+class site_apt::preferences::python_cryptography {
+
+  apt::preferences_snippet { 'python_cryptography':
+    package  => 'python-cryptography python-openssl python-pyasn1 python-setuptools python-pkg-resources python-cffi',
+    release  => "${::lsbdistcodename}-backports",
+    priority => 999;
+  }
+
+}

puppet/modules/site_mx/manifests/init.pp

@@ -19,4 +19,6 @@ class site_mx {
   include ::site_check_mk::agent::mx
   # install twisted from jessie backports
   include ::site_apt::preferences::twisted
+  # install python-cryptography from jessie backports
+  include ::site_apt::preferences::python_cryptography
 }