This creates a new class for the stats interface and furtherly
configures it to also use the certificates that are provided by
certmonger (via the internal_certificates_specs variable).
Note that the already existing haproxy_stats_certificate still works and
will take precedence if it's set.
bp tls-via-certmonger
Change-Id: Iea65d91648ab13dbe6ec20241a1a7c95ce856e3e