less misleading diagram that describes vCenter HA

- current diagram has following misleading moment: it states that
  nova-network and nova-compute has active and standby working modes,
  which is not true
- rename image file to vcenter-ha-reference.png, because it
  describes HA architecture, which is a part of reference architecture

Change-Id: I0ba89d2d5c097984c21ec31cbf3771d21be62c26
Merge "less misleading diagram that describes vCenter HA"