Feat/use claudie http proxy when building cluster in hetzner

[JKBGIT1]

closes https://github.com/berops/claudie/issues/783 https://github.com/berops/claudie/issues/1272

This PR adds support for HTTP proxy in kube-eleven. The default workflow is as follows. Claudie utilizes an HTTP proxy (http://proxy.claudie.io:48213) only when a user wants to build a K8s cluster that contains at least 1 node from the Hetzner Cloud provider. If the k8s cluster doesn't have any nodes from the Hetzner Cloud provider Claudie won't use an HTTP proxy in kube-eleven to build the desired k8s cluster.

The usage of an HTTP proxy is customizable. A user can turn off the HTTP proxy usage by setting HTTP_PROXY_MODE to off. This means Claudie won't use the HTTP proxy at all (even when there are nodes in the K8s cluster from the Hetzner cloud provider). On the other hand, the user can turn on the HTTP proxy usage whenever building a K8s cluster (even when there aren't any nodes in the K8s cluster from the Hetzner cloud provider) by setting HTTP_PROXY_MODE to on.

Besides that users can use their own HTTP proxy by setting HTTP_PROXY_URL to point to their proxy URL.

[bernardhalas]

Probably we should mention the use of value HTTP_PROXY_MODE=default in docs/http-proxy/http-proxy.md.