dashboard: Deprecated fqdn parameter to horizon

Specifying fqdn to horizon was the old way to specify ALLOWED_HOSTS entries.
It has been deprecated, and now one should rely on $allowed_hosts.

Note: There is no deprecation warning, since there was no way to specify
the fqdn value, it was bound to `$api_eth`. This change is transparent
to the user
Merge pull request #667 from enovance/deprecate_horizon_fqdn

dashboard: Deprecated fqdn parameter to horizon