Add ssl key and cert when @ssl = 'required', not only 'yes'

File: https://github.com/mxhero/puppet-dovecot/blob/master/templates/conf.d/10-ssl.conf.erb#L15

They `ssl` parameter takes the following parameters: `yes, no, required`, but it seems like you're only supplying the `ssl_cert` and `ssl_key` data when `@ssl =~ /yes/` and not also when `required`.
