Increase dashboard installation introduction clarity

Moved content associated with the administration guide
to the "next steps" section of the Horizon install guide.

Added content to the Horizon installation introduction
to increase clarity, and match the OpenStack
installation documents. Unified xml file header.

Change-Id: I3a1f9fc1e1515d22a6b3e7ddc19df1db3ecfd884
backport: none
Implements: blueprint installation-guide-improvements
Merge "Increase dashboard installation introduction clarity"