Add missing mysql_install_db command to Install Guide

As reported to Launchpad, the command "mysql_secure_installation"
was not available but was after running "mysql_install_db".
Changed the instructions to represent this and explained why.

Change-Id: I078d6c44d390a1f6a1a704bbae1a561655365871
backport: havana
Closes-Bug: 1260862
Merge "Add missing mysql_install_db command to Install Guide"