MacOS: when launched within IDE (not run from source) KUBECONFIG env variable is not available

[adietish]

related to redhat-developer/intellij-kubernetes#826

Steps:

1. ASSERT: launch plugin as binary or from source without setting KUBECONFIG in the launch config (env vars)

Result:
The KUBECONFIG env variable is not available to the kubernetes-client in the plugin.

[adietish]

fixed by code provided in redhat-developer/intellij-common#263 and a call to it in this plugin.