fix(k8s-gke): disable basic auth for K8S
fix(k8s-gke): disable basic auth for K8S

We use K8S 1.19 and going to use only newer versions later.
Then, basic authentication is deprecated in GKE and
has been removed in GKE 1.19 and later.

So, stop using basic auth specifying "--no-enable-basic-auth" option.
Also, remove uage of "--username" one which is mutually exclusive
with the above mentioned one.

Link: https://cloud.google.com/kubernetes-engine/docs/how-to/
                api-server-authentication#legacy-auth

scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests