Use a shared session key among Horizon nodes

This change adds a configuration setting for the Horizon session key. If
present, this key is set into all Horizon nodes, so that they can share
sessions.

Change-Id: I94f46f3adb8fa26965959d5e8c6473eff1b4591c
Closes-bug: 1403611
(cherry-picked from commit b5fbb9a673abeedf00c3632f26e31e2d3b18febc)