Unable to use HTTP_PROXY environment variable for proxy setting

[spnowkif]

Hi, the istio plugin doesn't provide a convenient way to set the proxy via an environement variable like HTTP_PROXY in the same way the kubernetes plugin allows.

This is the local config setup for the istio-plugin - https://github.com/chaostoolkit-incubator/chaostoolkit-istio/blob/master/chaosistio/__init__.py#L65-L69

The kubernetes plugin allows the proxy to be set from the HTTP_PROXY environment variable - https://github.com/chaostoolkit/chaostoolkit-kubernetes/blob/master/chaosk8s/__init__.py#L70-L74

Could the istio plugin use the same approach as the kubernetes plugin?