os-net-config should check for duplicate IPs

[Dan Sneddon]

When os-net-config configures static IPs for a host, it does not check to see if that IP is in use on the network. This causes failures which are difficult to diagnose in a TripleO deployment. To prevent these mystery failures, os-net-config should check to see if the IP is in use on the network before configuring the IP on the host.

This can be done with the 'arping' utility:

arping -D -I <interface-name> -c 2 <IP-ADDRESS-TO-CHECK>

The arping utility can be run on an interface even before an IP has been configured on that interface, so we can check to ensure that the desired IP is free.