document "vCenter full scale HA support"

- describe how service behave on failure in Multi-node HA mode
- update screenshot in Reference Architecture which shows that
  nova-network and nova-compute can run on distinct controllers
- add limitation about security groups which are not support by
  nova-network with vCenter
- add limitation that vCenter 5.1 and later are supported
- add limitation about that we cannot select in which vCenter cluster
  virtual machines are started
- remove limitation about sparse HA support
- rename flatDHCP to FlatDHCP

Change-Id: I1b238334caf524e7c903d74de59282c4e1f5978d
Implements: blueprint vcenter-documentation-update-fuel-5.1