It is useful to apply the storage testsuite also to "external" (=
out-of-tree) storage drivers. One way of doing that is setting up a
custom E2E test suite, but that's still quite a bit of work.

An easier alternative is to parameterize the Kubernetes e2e.test
binary at runtime so that it instantiates the testsuite for one or
more drivers. Some parameters have to be provided before starting the
test because they define configuration and capabilities of the driver
and its storage backend that cannot be discovered at runtime. This is
done by populating the DriverDefinition with the content of the file
that the new -storage.testdriver parameters points to.

The universal .yaml and .json decoder from Kubernetes is used. It's
flexible, but has some downsides:
- currently ignores unknown fields (see https://github.com/kubernetes/kubernetes/pull/71589)
- poor error messages when fields have the wrong type

Storage drivers have to be installed in the test cluster before
starting e2e.test. Only tests involving dynamically provisioned
volumes are currently supported.