Clarified database configuration steps for Block Storage

I clarified the database configuration steps for Block Storage for
consistency with the associated patch applied to Havana. I also added
a note for Ubuntu users to add the '[database]' section header if not
included with the default "cinder.conf" file. This patch covers both
the controller and storage nodes.

Change-Id: I6ae7072b2a98db00a6367213721f883072f8348b
Add section for LVM driver

The config reference was missing a section for the default driver,
LVM. This adds a new section, with the config options needed to use
it and fixes the incorrect mention in the section lead-in.

Closes-Bug 1208205

Change-Id: Idefab5dbd69c37d637bc2a3f75880d8e341379ee
Merge "Clarified database configuration steps for Block Storage"
Merge "Add section for LVM driver"