Install Guide: Correct Neutron Section Tree

This commit moves around a couple of pieces of the install guide
neutron documentation. The actuall root install section now no
longer refers to the network node; that now has its own subsection.
Additionally, the instructions for initializing the database are
now in the root section.

This is the final commit in the series.

Change-Id: I94cd2b1cac9fae9936110ecaedf13c6eb895a2a8
backport: havana