Remove confusing message about mysql credentials

The Glance installation material contained a note indicating that if
connecting to MySQL fails the user should use the IP address of the
database server instead when specifying the credentials.

Besides being confusing in its wording this note referred to a situation
that the user could not possibly become aware of until starting the
Glance services later in the chapter.

Additionally no such warning is included elsewhere in the guide as its
presumed (via specification in the prerequisites up front) that the user
has set up their /etc/hosts files such that the controller host name will
resolve.

Change-Id: Ic5e6e1820628b0854effe7594b09610d030d0573
Closes-Bug: #1251458
backport:havana
(cherry picked from commit 426429b109d6b946745f3c15de949e59050ba5c1)
Update headings, edits for consistency/clarity/ Install Guide

Partial-Bug: #1250515

backport: havana

Change-Id: I11117af7e13966f70e0050f96a7b2a1c4bcb59ca
author: diane fleming
(cherry picked from commit 4247c757448f66f6ffc05501f852ccceba377ae6)
Merge "Remove confusing message about mysql credentials" into stable/havana