Fix the installation bug in startup.sh file

The k8s can not deployed properly when status of OS's swap is on.
It mainly because that the "sudo swapon -a" operation should be
invoked after the "kubeadm join xx" command, but not before it.
So, in this patch, the sequency of 2 commands were re-ordered for
fixing it.

Signed-off-by: Jingzhao.Ni <Jingzhao.Ni@arm.com>
Change-Id: I7bac839a1b77a76f536da35989741776aefb5929