Start running go vet ./... in CI

[klingerf]

We can use go vet to detect go code issues that are not caught by the compiler. If we add this to CI, then it should prevent regressions going forward. We'll also need to fix these issues as part of that branch:

$ go vet ./...
pkg/k8s/kubectl.go:88: arg kctl.ProxyPort in printf call is a function value, not a function call
pkg/k8s/kubectl_test.go:80: arg kctl.ProxyPort in printf call is a function value, not a function call
pkg/k8s/kubectl_test.go:108: arg kctl.ProxyPort in printf call is a function value, not a function call
exit status 1
pkg/shell/shell_test.go:103: arg output for printf verb %s of wrong type: *bufio.Reader
exit status 1
proxy-init/integration_test/iptables/http_test.go:198: arg resp for printf verb %s of wrong type: *net/http.Response
exit status 1
proxy-init/integration_test/iptables/test_service/test_service.go:50: arg amITheProxy for printf verb %s of wrong type: bool
exit status 1
web/main.go:67: the cancel function returned by context.WithTimeout should be called, not discarded, to avoid a context leak
web/main.go:41: arg kubernetesApiHost for printf verb %s of wrong type: *string
web/main.go:45: arg kubernetesApiHost for printf verb %s of wrong type: *string
exit status 1