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 "Fix wrong description for nova image-create"