Introducing --azure-dns argument which associates a FQDN to the
created public IP resource for the VM.
e.g. a value like 'foo' will be later used as IP 'foo.westus.azure.cloudapp.net'.
Also modifying `ip` subcommand to operate on FQDN, where possible.
Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>