Support reading KubeConfig from KUBECONFIG env var

[aliok]

We don't currently support reading KubeConfig from KUBECONFIG env var and because of that the OLM scorecard tests fail. See operator-framework/community-operators#1192

We support passing the masterUrl and kubeconfig as command line arguments only.

BTW, I think we need to skip having 0.12 on OperatorHub because we wouldn't make a new release only for this ticket.

I closed the ticket at Operator Hub operator-framework/community-operators#1192

[houshengbo]

@aliok We can skip 0.12.0, but we can have 0.12.x as we are/will be releasing as a INDEPENDENT project. If this is a crucial fix for operator, it is also an example that operator should backport to the 0.12 release.