Invalid axis2.xml clustering config parameters

[lasanthaS]

Description:
In Kubernetes membership scheme README.md file, KUBERNETES_MASTER_USERNAME and KUBERNETES_MASTER_PASSWORD config properties are mentioned but not used. Instead they need to be updated as follows.

KUBERNETES_MASTER_USERNAME => KUBERNETES_API_SERVER_USERNAME
KUBERNETES_MASTER_PASSWORD => KUBERNETES_API_SERVER_PASSWORD

Suggested Labels:
Bug

Affected Product Version:
1.0.0

Related Issues:
https://github.com/wso2-attic/kubernetes-artifacts/issues/36