Support automatic SSL certificates from Let's Encrypt

[thebe14]

Would be great to have an automatic SSL certificate out of the box, by just specifying the domain name on which I want to publish CKAN. Will probably have to also provide (in the configuration) an email address to be used when requesting the certificate. Let's Encrypt can supply the certificate, but it will have a short lifespan (90 days), therefore a mechanism to automatically update it is also required.