Renaming chapter 3 to, "Add the Identity service"

This patch renames chatper 3 to "Add the Identity service" as
a part of the Install Guide Improvements.

Change-Id: Ia987c47ab6fbfc9497824a95462060b3fd73915e
Implements: blueprint installation-guide-improvements
Merge "Renaming chapter 3 to, "Add the Identity service""