Update horizon manifest to use new ssl variable names

A recent change [0] to puppet-horizon renamed horizon_cert and
horizon_key to use different names. This commit updates puppet-tripleo
to use the appropriate names.

[0] https://review.opendev.org/#/c/758041/6

Change-Id: Ie479c69b77c67425188160a8af8c933ad6db79f4

manifests/profile/base/horizon.pp

@@ -104,8 +104,8 @@ class tripleo::profile::base::horizon (
     class { 'horizon':
       cache_server_ip => $horizon_memcached_servers,
       neutron_options => $neutron_options_real,
-      horizon_cert    => $tls_certfile,
-      horizon_key     => $tls_keyfile,
+      ssl_cert        => $tls_certfile,
+      ssl_key         => $tls_keyfile,
     }
   }
 }