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.

Please note that opnfv-scenario-requirements.yml contains version and
the refspec of the corresponding change in releng-xci-scenarios repo
so this change must be verified properly and if it passes CI, it
needs to be amended, updating k8-calico-nofeature scenario's version
to master and submitted. And then the change in releng-xci-scenarios
can properly be verified.

https://gerrit.opnfv.org/gerrit/#/c/releng-xci-scenarios/+/68418/

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

JIRA: XCI-20

Change-Id: I1f851d20f5f7bc256199303581b05659091fddf9
Signed-off-by: Fatih Degirmenci <fdegir@gmail.com>