First pass of vCenter information

This includes the following pieces:
- Section in pre-installation about prerequisites and planning considerations
- Section in ref-arch that explains the vCenter implementation
- Section in user-guide with specific instructions
  for deploying MOS with vCenter integration.
- Section in operations about using vCenter
- Entries in terminology for major components

The pre-install guide is being restructured in a different CM; the
location of the vCenter information needs to be established in that
context.

The current ref-arch guide does not have a well-written and complete
section about hypervisors and how they fit into the "big picture". I
put the vcenter section in the general vicinity of other existing
material that will be part of that discussion.

The user-guide is being restructured in a different CM; the location of
the vCenter information needs to be established in that context.

The ops-guide is currently a somewhat random assemblage of information;
it will be structured better in a later release.  The vCenter section
may become a subsection of another larger section but this piece does
not currently exist.  It seems best to put vCenter in as a separate
section for now.

Change-Id: I55bc68e7910c29f31fecf79fb18bcaae23a17068
(cherry picked from commit b5a607d3ad08ed8bf0b5002993e6468781435979)
Merge "First pass of vCenter information" into stable/5.0