horizon::vhost_extra_params is already configured in
puppet/services/horizon.yaml, and users can change the value with
HorizonVhostExtraParams parameter.
Docker deployments didn't have HorizonVhostExtraParams taken in account
since we were overriding with Hiera. This patch fix it.
Closes-Bug: #1749627
Change-Id: I77f1312112c7f613d795242060709082ef72f150