Add python-cryptography from backports

This commit makes sure that a recent enough python-cryptography version
is installed before the first salt highstate run.

bootstrap.sh.tmpl

@@ -1,5 +1,10 @@
 #!/bin/sh -
 
+echo "deb http://httpredir.debian.org/debian jessie-backports main" >/etc/apt/sources.list.d/backports.list
+
+apt-get update
+apt-get install -y -t jessie-backports python-cryptography
+
 # We just download the bootstrap script by default and execute that.
 if [ -x /usr/bin/fetch ]; then
     /usr/bin/fetch -o - https://raw.githubusercontent.com/saltstack/salt-bootstrap/stable/bootstrap-salt.sh | sh -s -- "$@"