Refactor k8s scenario implementation

Refactor k8s scenario implementation

This change updates the scenario logic for basic k8s
scenarios that enables the use of various network plugins.

The change simply stops copying k8s-cluster.yml over but instead
operates on the existing/original k8s-cluster.yml and updates
kube_network_plugin in place.

This makes it easier to maintain the scenarios and more importantly
unblocks us from bumping kubespray and kubernetes versions.

In order to verify this change properly, below change first must be
merged.

https://gerrit.opnfv.org/gerrit/#/c/releng-xci/+/68423/

JIRA: XCI-20

installer-type:kubespray
deploy-scenario:k8-calico-nofeature

Change-Id: Icf78a4fc693c659290714e666a2941b6f6e9487a
Signed-off-by: Fatih Degirmenci <fdegir@gmail.com>