In repos that have a test/e2e, that test suite should be run
separately because it depends on a running cluster.

This is an unmodified copy of kubernetes/hack/verify-shellcheck.sh
revision d5a3db003916b1d33b503ccd2e4897e094d8af77.

These are the modifications that were necessary to call this outside
of Kubernetes. The support for excluding files from checking gets
removed to simplify the script. It shouldn't be needed, because
linting can be enabled after fixing whatever scripts might fail the
check.

By default this only tests the scripts inside the "release-tools"
directory, which is useful when making experimental changes to them in
a component that uses csi-release-tools. But a component can also
enable checking for other directories.

This enables testing of other repos and of this repo itself inside
Prow. Currently supported is unit testing ("make test") and E2E
testing (either via a local test suite or the Kubernetes E2E test
suite applied to the hostpath driver example deployment).

The script passes shellcheck and uses Prow to verify that for future
PRs.

The travis.yml is now the only place where the Go version for the
component itself needs to be configured.

Using the same (recent) Go version for all Kubernetes versions can
break for older versions when there are incompatible changes in Go. To
avoid that, we use exactly the minimum version of Go required for each
Kubernetes version. This is based on the assumption that this
combination was tested successfully.

When building the E2E suite from Kubernetes (the default) we do the
same, but still allow building it from elsewhere.

We allow the Go version to be empty when it doesn't matter.

While switching back and forth between release-1.13 and release-1.14
locally, there was the problem that the local kind image kept using
the wrong kubelet binary despite rebuilding from source. The problem
went away after cleaning the Bazel cache. Exact root cause unknown,
but perhaps using unique tags and properly cleaning the repo helps.

If not, then the unique tags at least will show what the problem is.

Instead of always using the latest E2E tests for all Kubernetes
versions, the plan now is to use the tests that match the Kubernetes
version. However, for 1.13 we have to make an exception because the
suite for that version did not support the necessary
--storage.testdriver parameter.

The temporary fork was merged, we can use the upstream repo again.

There's nothing to configure at the moment, we can use the share
prow.sh without additional configuration.