Replaces Certbot with Lego for certificate retrieval and renewal.
Lego includes support for DNS ACME Challenges.
Adds ssh-client to LetsEncrypt and ssh-server to HAProxy to allow both
the transfer of Let's Encrypt certificates to the HAProxy container and
to enable live updating of HAProxy certifices using the HAProxy API
exposed on the local HAProxy socket.
Implements: blueprint letsencrypt-https
Change-Id: I737e1ce5bfc37d0703879c8272a9e915084c5ca6
d424a63d60