Don't use latest stable version for minikube

Sometimes, minikube lags behind kubernetes which can lead
to a breakage of the test suite, since minikube will fail to start
with the latest kubernetes version.
See for example [this issue](kubernetes/minikube#4371).

This change defaults to leave the decision of which k8s version to use,
to minikube itself. This is defined in:

https://github.com/kubernetes/minikube/master/pkg/minikube/constants/constants.go

However, if one really desires it is still possible to pass

`--kubernetes-version=X.Y.Z`

to minikube initialization start command via an environment variable
before invoking the test suite:

export $MINIKUBE_ARGS="--kubernetes-version=X.Y.Z"

This allows ofcourse passing other flags to minikube also.

scripts/kube-init.sh

@@ -44,10 +44,19 @@ else
 fi
 docker --version
 
-# Get the latest stable version of kubernetes
+# Get the latest stable version of kubernetes, this is not always what minikube
+# installs per default
+# See: https://github.com/kubernetes/minikube/master/pkg/minikube/constants/constants.go
 K8S_VERSION=$(curl -sS https://storage.googleapis.com/kubernetes-release/release/stable.txt)
 echo "K8S_VERSION : ${K8S_VERSION}"
 
+# You can pass variables to minikube using MINIKUBE_ARGS
+# If using tox you can export TOX_TESTENV_PASSENV.
+# For example, you can run:
+# $ export TOX_TESTENV_PASSENV="MINIKUBE_ARGS=--kubernetes-version=1.X.Y"
+# now tox will run minikube with the specified flag
+MINIKUBE_ARGS=${MINIKUBE_ARGS:-""}
+
 echo "Starting docker service"
 sudo systemctl enable docker.service
 sudo systemctl start docker.service --ignore-dependencies
@@ -80,7 +89,7 @@ export MINIKUBE_DRIVER=${MINIKUBE_DRIVER:-none}
 # Used bootstrapper to be kubeadm for the most recent k8s version
 # since localkube is depreciated and only supported up to version 1.10.0
 echo "Starting minikube"
-sudo minikube start --vm-driver=$MINIKUBE_DRIVER --bootstrapper=kubeadm --kubernetes-version=$K8S_VERSION --logtostderr
+sudo minikube start --vm-driver=$MINIKUBE_DRIVER --bootstrapper=kubeadm --logtostderr $MINIKUBE_ARGS
 
 MINIKUBE_OK="false"