Fix image reference in basic-install.

When renaming Quantum to Neutron and moving an impacted shared image to
"common" it looks like the relative path used to refer to it wasn't
quite right. I have updated it to go down the additional level in the
directory structure and the book now builds.

Change-Id: I116e9d74ffd5ec2ced17f1168ccc22a22305799e
Merge "Fix image reference in basic-install."