Automatic merge from submit-queue
Making DHCP_OPTION_SET_ID creation optional
Reason: We have a pre-configured VPC in AWS. `kube-up.sh` should not making changes to the VPC DHCP option if there's already DHCP options configured.
PR Changes: When `DHCP_OPTION_SET_ID` is given in environment variable, kube-up.sh will skip the `DHCP_OPTION_SET_ID` creation.