Kubernetes also requires 1.12. We pick the latest stable release
for CI builds.

update to Go 1.12.4

How a repo does vendoring is detected based on the presence of
Gopkg.toml.

The vendor check with `dep` was all done locally, but the
corresponding check for `go mod` requires network access. The check
therefore gets skipped when running in the Prow CI in situations where
we are sure that it isn't needed (for example, in a periodic job).

build.make: allow repos to use 'go mod' for vendoring

By moving the code into a separate function, other CSI drivers have a
chance to overwrite it. For the hostpath driver itself we need the
ability to set the driver name depending on which revision is getting
installed.

more flexible test driver config

with specific patch versions that kind 0.4.0 supports. Also, feature
gate setting is only supported on 1.15+ due to
kind.sigs.k8s.io/v1alpha3 and kubeadm.k8s.io/v1beta2 dependencies.

Add 1.15 jobs