Add `associate_public_ip_address` flag

Packer CloudStack plug-in always try to associate a public IP address. Some CloudStack instances do not allow users to associate public IP address to virtual machines, leading Packer to fail with message "Failed to associate public IP address...".

Amazon EBS plugin for instance has a flag `associate_public_ip_address` allowing sources not to be associated to public IP address. Can we have this flag for CloudStack too?