Add instructions for running kubectl directly from the bastion host

[thinhha]

In https://github.com/GoogleCloudPlatform/gke-anthos-holistic-demo/blob/master/README.md

Around when you define the k proxy through the bastion host:

alias k="HTTPS_PROXY=localhost:8888 kubectl"

Maybe indicate that you can also ssh directly into the bastion using the GCP console, set up kubectl using the output of:

echo $(terraform output get_credentials)

Then skip the k alias step and continue from there.