Update to 1.8.0.
Update to 1.8.0.

Adds various library changes:

 * Uses slim debian:stretch image for smaller images.
 * Changed to PCRE2, as it is now supported by 1.8.x.
 * Changed from LibreSSL to OpenSSL, in order to support the async
   feature. LibreSSL support was not always present in the current
   HAProxy version, and typically required older LibreSSL versions to
   compile, thus negating the reason for switching to LibreSSL. With
   addition of the async feature, parity is lost between the two
   libraries.

aasmith/docker-haproxy,aasmith/docker-haproxy