Allow testing of the CLI against configurable Kubernetes versions

[advdv]

We want to make the minikube Kubernetes version we test against configurable through a environment variable

Expected Behavior

KUBE_VERSION=1.8.0; ./make.sh test should switch to testing against a 1.8.0 minikube vm

Actual Behavior

It tests against a hardcoded Kubernetes version (1.7.5)

Steps to Reproduce the Problem

1. ./make.sh test

Specifications

• Version 0.6.0 dev branch
• Platform: MacOS
• Subsystem: High Sierra

Anything else we need to know?

We are probably not able to provide full support but we should be able to test and document what kube versions are supported