Add a parameter to vhost::proxy to deny by verb

Add the $denied_http_verbs argument to the nginx::vhost::proxy defined
type to allow HTTP verbs to be denied. It accepts an array of verbs to
block. It causes Nginx to return a 403.
Add explanatory comment to @denied_http_verbs

Add an explanatory comment to the code that produces a regular
expression from the @denied_http_verbs array as it wasn't clear.
Merge pull request #20 from alphagov/add-denied-http-verbs

Add a parameter to vhost::proxy to deny by verb