Information about VMware vCenter and NSX features is updated.

Following documents are updated:
- Terminology: links in the 'nsx' term added
- Planning guide:
-- Choose Network Topology: information about nsx added
-- Nodes and Roles: notes added
-- Preparing vSphere Integration: notes and limitations added
- Reference Architecture: notes added, HA setup diagram updated
- User Guide: screenshots updated, vcenter-deploy.rst updated,
  7310-nsx-deploy.rst, add screenshot with filled NSX settings

I added a link to the vCenter page for assigning roles.  The
screen shot was too small to really see the verbage about not
assigning a Controller node so I increased the size of that image
and others in the vicinity.

Implements: blueprint vcenter-documentation-update-fuel-5.1
Change-Id: Ibcb5c1b7cd2a286e2239d17d87ea49eb10930964