Add another clarifying note on nova-net install

We had another bug report about the nova-network install section.

This patch adds another very explicit,
clarifying note to ensure people are installing
packages in the right place.

Change-Id: Ifc48813af9ec1cf8a87aab96c3f95c8d474595c9
Closes-bug: 1261620
Merge "Add another clarifying note on nova-net install"