How to set http_proxy, https_proxy with docker provisioner

## Use Case
I our company, we need to use coorporate proxy to access on internet.
So, litmus failed when it provision centos7 because of http_proxy and https_proxy is not set when It try to install openssh-server.

## Describe the Solution You Would Like
How to set proxy parameters `http_proxy` and `https_proxy` before it install openssh-server, etc ...


