I've created my own centos 7 image from kickstart but the instance doesn't broadcast dhcp requests (verified using tcpdump.)  If I launch an instance using the image you provide (CentOS-7-x86_64-GenericCloud-1511.qcow2) it does broadcast a dhcp request and then comes up on the network.  My kickstart file does indicate "network --bootproto=dhcp --onboot=on", but I want to compare it to yours which is working as I would expect.