Implement `OSCONFIG` env similar to KUBECONFIG

[gtema]

Kubernetes tools support setting the KUBECONFIG variable to the list of paths (i.e. KUBECONFIG=~/.kube/config:~/.kube/config.1). It gives possibility to merge all configs into a single virtual config file while splitting different config elements separate (i.e. company connections, private connections, etc). While we support --os-cloud-config vars it would be also reasonable to provide the mechanisms similar to kubernetes tools.