Update kubeadm installation steps

[sunilfernandes]

The instructions on domain-1-cluster-setup/kubeadm.md will need to be updated as the below commands are failing

Failing commands :

apt-get update && apt-get -y install docker-ce docker-ce-cli

Error:

root@server1:~# apt-get update && apt-get -y install docker-ce docker-ce-cli
Get:1 http://mirrors.digitalocean.com/ubuntu focal InRelease [265 kB]
Hit:2 http://mirrors.digitalocean.com/ubuntu focal-updates InRelease
Hit:3 http://mirrors.digitalocean.com/ubuntu focal-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu focal-security InRelease
Fetched 265 kB in 1s (262 kB/s)
Reading package lists... Done
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package docker-ce is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'docker-ce' has no installation candidate
E: Unable to locate package docker-ce-cli

Fix:

Add instructions from https://docs.docker.com/engine/install/ubuntu/

Specifically setting up the stable repo steps