Current logic does not allow horizon backend to listen on https
(`horizon_enable_ssl`) if external loadblanacer serves TLS
(`horizon_external_ssl`).
It basically forces backend to listen on plain http in this case which
does not make any sense. It should be possible to enable TLS on both
loadbalancer and horizon backend.
Additionally, with this patch, role defines a proper
HTTP_X_FORWARDED_PROTO header value(it's included in
`horizon_secure_proxy_ssl_header` and
`horizon_secure_proxy_ssl_header_django` and can be set to 'http' or
'https') based on whether external load balancer listens on https
(`horizon_external_ssl`)[1].
For example if loadbalancer listens on https and backend on http,
HTTP_X_FORWARDED_PROTO should be set to 'https'. Otherwise horizon will
respond with redirection to http.
[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto
Change-Id: I7706e52c01b3f0d72ea383a0476045e606078cff
c92f45e3af