Document nova-network single-host mode with vCenter
Document nova-network single-host mode with vCenter

- Document important fact that nova-network works in single-host mode
  when vCenter is enabled and what might happen if it crashes

Change-Id: I8a78c49aaa8eb035535c911ecc9d141ba93bbf0e
Implements: blueprint vmware-dual-hypervisor
ogelbukh/fuel-docs,ogelbukh/fuel-docs,stackforge/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs,phillipsio/fuel-docs,phillipsio/fuel-docs,stackforge/fuel-docs,phillipsio/fuel-docs,phillipsio/fuel-docs