Add a new boolean configuration option for copying of SSL certificates.

Introduced a new option to disable or enable copying SSL related files like certificates, keys, etc.

For example if there is a shared wildcard certificate there are a lot of copies created of the same certificate which will mess up the Nginx configuration folder. As discussed in issue #80 the default system directory should be used.

Example of usage changeset:
class { 'nginx':
    copy_ssl_files    => false,
  }

nginx::resource::vhost { 'name':
    ensure           => present,
    ssl              => true,
    ssl_cert         => '/etc/ssl/certs/file.pem',
    ssl_key          => '/etc/ssl/certs/file.key',
    ssl_dhparam      => '/etc/ssl/certs/file.dhparams.pem',
  }

The 'copy' option is enabled by default for backward compatibility. It might be a solution to remove this option in one of the upcoming releases.