`horizon_enable_ssl` is responsible for enabling TLS on horizon backend.
It defaults to `haproxy_ssl` which is generally used to enable TLS on
haproxy frontends.
It is more reasonable to disable it by default as it's done for other
services.
This patch does not change current behavior in gating as backend TLS
works only with horizon_external_ssl=False(while it's set to True by
default).
It also does not affect behavior of horizon's haproxy frontend
encryption.
Change-Id: I8f207426c9dc5bcefdec42c0bfc0f5f0376509a3
9c07e79890