Disable multi_host for nova-network with vCenter

- In HA setup with vCenter nova-network runs on OpenStack controller
  nodes.  We must be sure that nova-network service has single running
  instance among all controllers, otherwise VMs would see multiple
  gateways/DHCP servers in their network.  By default multi_host is
  enabled in HA mode and nova-network will run on every controller.

Partial implements: blueprint vcenter-hv-full-scale-support

Change-Id: I9cb7e1deaa0e533ce81e5f24159600493b25196f
Merge "Disable multi_host for nova-network with vCenter"