add new section "Preparing for NSX integration"

- briefly describe installation steps
- describe configuration steps that are important:
  * user has to write down Transport zone UUID and Gateway Service UUID
  * user has to choose transport type for gateway node, he must use the
    same transport type on Settings tab
- provide URLs that describe detailed process of installation and
  configuration NSX cluster
- provide instructions how to host NSX specific packages on Fuel Master
  node
- limitation: only NSX 4.0 is supported
- limitation: only KVM or QEMU are supported as hypervisor options
- limitation: entities that were created in NSX cluster are not flushed
  on "Reset Environment" or "Delete Environment" actions ("Reset" and
  "Delete" buttons on the Actions tab)

Change-Id: Ia30204479ac5ef3210a008561d7a338a8bd314a5
Implements: blueprint update-release-notes-fuel-5.1