Add support for Diffie-Hellman (SSL) parameters in VHOST resource.

Introduced optional parameter for SSL Diffie-Hellman support.

Nginx documentation:
This directive specifies a file containing Diffie-Hellman key agreement protocol cryptographic parameters, in PEM format, utilized for exchanging session keys between server and client. 
Syntax: 	ssl_dhparam file 

Example puppet implementation:
ssl => true,
ssl_cert => 'puppet:///modules/modulename/ssl.hostname.crt',
ssl_key => 'puppet:///modules/modulename/ssl.hostname.key',
ssl_dhparam => 'puppet:///modules/modulename/dh.hostname.pem',