Enhancements to the vCenter docs

Pre-install/7000-vcenter-plan -- add link to vCenter installation docs and
summarize the requisite steps, with screen shots.

Ref-arch/logical-setup/7000-vcenter -- corrected info about vCenter driver
for OpenStack. Added reference architecture and network topology diagrams

User-guide/vcenter.rst -- Added complete descriptions with screen shots

General: left-justified all screen shots to match the rest of the guide
and to avoid a tools bug that is sometimes doing odd things with graphics
for which :align" is specified.

relnotes/050-known-issues -- added LP to vCenter HA limitation

Closes-Bug: 1325690

Change-Id: I1ff4369a8ab03ff91f3e3c9cdebfde8a60d56808
(cherry picked from commit 5744c62ca6bb51965efb32230a7fd31f9408c5f4)