Adds a paragraph about the intended audience

- For each audience, end user and admin user, add
a paragraph in the first section about who the book
is intended for.
- Also edited Conventions and Doc History sections

Change-Id: I89fdbd9b62515357e434b7a27cc26455e1541efc
Fix wrong description for nova image-create

In the subsection "Create an image (nova)", there is a sentence said "You
cannot create a snapshot from an instance that has an attached volume",
which must be wrong. Currently, nova supports creating image from an
instance with attached volumes.

Change-Id: I811a845b8ee028ab83a12af03364930a0394e18d
Closes-Bug: #1341348
Merge "Adds a paragraph about the intended audience"
Merge "Fix wrong description for nova image-create"