loadbalancer: use http mode for Horizon haproxy config

loadbalancer: use http mode for Horizon haproxy config

The haproxy configuration for horizon does not have 'mode http' set.
This proxy needs to be in http mode since it is using a cookie for
persistence. The default section has 'mode tcp', which is fine, but
horizon proxy needs to override this setting to get http mode. Without
this, you will likely see an error like this:

[WARNING] 238/115010 (13878) : config : cookie will be ignored for proxy
'horizon' (needs 'mode http').'

Closes BZ-1257687

Change-Id: I397986ea022f47a33a5210696752509f4a2731a5