Set floating IP address to dns if available

HI,

I found that a private IP address is set to the `adress` of `dns` even when a floating IP address is assigned to a MicroBOSH. This `dns.address` value will be passed to VMs created by the MicroBOSH and at last it will be saved in /etc/resolve.conf. When you update the stemcell and its private ip is changed, VMs already deployed continuously cannot resolve `.microbosh` domain names.
This commit fixes the problem by setting the given floating IP address to `dns.address`.