Fix for UPDATE_HORIZON_CONFIG listed as part of Icehouse in the docs

The UPDATE_HORIZON_CONFIG setting was only added in Juno, the docs
incorrectly state that it is part of the Icehouse Release.

Change-Id: Id889b7c87c0a8c4347acf2f27af76ec9a4c1e6bf
Closes-bug: #1328871
Merge "Fix for UPDATE_HORIZON_CONFIG listed as part of Icehouse in the docs"