Support for custom SSL certificate and key (by path)

[mamercad]

There are options for providing st2web_ssl_certificate and st2web_ssl_certificate_key, which work just fine. It would be nice to alternatively provide paths to these files instead (versus the string contents). I'm currently working on a deployment which leverages a separate role to generate the certificate and key and it'd be convenient to be able to reference their paths (versus fetching them into strings to feed to the st2 roles).