Add user story about using networks into user-guide

[Denis Klepikov]

Add user story about using networks into user-guide
Lets describe networks and used IP ranges.
We have:
x.x.x.x/xx network1
y.y.y.y/yy network2
etc
and for our cluster we will use
network1 for this
network2 for this
etc

It will help customers to understand topology and helps them to use their networks for proper installation of the Fuel-master node.