Add option to disable SSL in Jetty

This patch introduces the optional parameter $disable_ssl, which
defaults to false. If set to true, the settings ssl-host and ssl-port
are completely removed from the Jetty section of the PuppetDB config
files.

This disables serving of HTTPS requests by PuppetDB, which can be useful
when SSL handling is offloaded to a reverse proxy server like Apache or
Nginx, as suggested in the PuppetDB documentation (see
http://docs.puppetlabs.com/puppetdb/1.2/connect_puppet_apply.html#option-a-set-up-an-ssl-proxy-for-puppetdb).