Now if clusterName is specified for kubebuilder, Build() method
will return kubeconfig containing only context for that cluster.
If no clusterName is specified kubeconfig will have contexts of
all clusters that builder can build. This behaviour is similar
to `kubeconfig get` command.
Relates-To: #460
Change-Id: Ib3bebf61bc60430d347ded41b2e31249b11126eb