Install Guide: Explictly mention controller node

Explain that these steps are run on the controller node.

Change-Id: I5a46cd632a61af1c3d35dcc3e1b0df495a356097
Closes-Bug: #1257139
Backport: havana
(cherry picked from commit d3164ebccf79e1b96ca2ad14e72348d534aa5f0d)
Merge "Install Guide: Explictly mention controller node" into stable/havana