prow.sh: better test filtering, junit merging, sanity testing, versioned configs

This separates which kind of tests to run (serial/parallel,
stable/alpha) from specifying what those tests are. Regular
expressions for choosing tests only need to be specified once now, the
actual focus and skip options are derived from that.

Merging all junit files into one while throwing away the tests that
we are not interested in increases load times in Spyglass. The
implementation in Go is not as fancy as a XLST style sheet or an
XPath-based filtering tool, but has the advantage that it works
without introducing additional dependencies.

csi-sanity testing is useful for CSI drivers.

There's no longer a need to explicitly provide a configuration and
deployment for the Kubernetes version that is being used for testing.
Instead, the "latest" settings and deployment are used as fallback.

Author: pohly