Some environments use a dedicated PKI for monitoring and metric collection.
This change allows to configure the serving certificate for stats independently
by setting `haproxy_stats_ssl_cert_path`, the default is to use the same cert.
Also client certificate authentication for stats can now be enabled by defining
a CA cert via `haproxy_stats_ssl_client_cert_ca`.
Change-Id: Ib8be6b7fc3dada9d20905b0f07d90ddce0335605