makefile: add clean-test as a pre-requisite to e2e

[surajssd]

If there is a dirty environment which had e2e run before, if you try
re-running e2e the tests fail.

This commit adds makes clean-test as a pre-requisite to e2e tests.

[krnowak]

I was about to ask if doing kubectl delete on a nonexistent resource make kubectl's exit status to be nonzero, but then I thought that if the tests passed then this change would be safe (as I don't have a k8s cluster at my hand for now to check it myself). But I see that there are no tests at all and that is because the branch you want to merge comes from your fork at your account. And there, CI was not set up at all. Could you create a PR from a branch that comes from our kinvolk clone and then close this one? With that done, we will get tests running.