Fix path to network interfaces configurations

This commit changes the path that is used as reference in the
documentation to one that is widely used in both ubuntu and debian. This
was made in order to avoid confusions (that actually happened while I
was trying out the devstack guide).

Change-Id: Ic3ce40816f79f9f522fc33ed8b4a574d7a57d586
Merge "Fix path to network interfaces configurations"