document multiple vCenter clusters support

- explicitly state that the Nova compute service can manage multiple
  ESX host clusters via single vCenter server
- update screenshot where vCenter hypervisor is selected, which shows
  how to specify multiple clusters
- remove limitation which states that single OpenStack environment can
  communicate with only one vCenter cluster

Implements: blueprint vcenter-documentation-update-fuel-5.1
Change-Id: I7da867f45b05c9d7dd0dbda31178f26422f49dc1