augment "Neutron with NSX topology" section

- we recommend to dispose NSX nodes in OpenStack Management network if
  possible
- rename Neutron_nsx_topology.png to _images/Neutron_nsx_l2.png that
  illustrates aformentioned topology
- otherwise NSX network must have L3 connectivity with OpenStack Public
  network; in this case Compute nodes must receive Public addresses
  (checkbox on Settings tab must be ticked)
- mention this in Known Issues for 5.1 release
- add figure for topology with L3 connectivity
- while here fix typo on figures ("Constrollers")

Change-Id: I6f7948a9bfa2b5c6e7cf6e8a3636d94f913a71d1
Closes-bug: #1367859