Implement cleanup functionality for nova-network OCF script

* implement nova_network_cleanup() function
* determine network segmentation type that nova-network uses (for now
  only FlatDHCPManager supported)
* depending on IP protocol version flush certain iptables chains
* try 3 times to remove all IP addresses assigned to 'flat_network_bridge'
  (usually called 'br100')
* call nova_network_cleanup() during start, we need to be sure that
  there are no garbage when we start
* call nova_network_cleanup() when nova-network failed in
  nova_network_stop(), we need to stop dnsmasq, flush IP addresses on
  'br100' bridge and iptables rules, so thath VMs would not see two
  gateways/DHCP servers in their network
* call nova_network_cleanup() after successfull service stop
* check that 'ip', 'iptables' and 'ip6tables' binaries are available

Change-Id: I098afe269e55a2809ca230b2d6c3ec60c669cb8c
Partial implements: blueprint vcenter-hv-full-scale-support
Merge "Implement cleanup functionality for nova-network OCF script"