initial Prow test job

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.

.prow.sh

@@ -0,0 +1,7 @@
+#! /bin/bash -e
+#
+# This is for testing csi-release-tools itself in Prow. All other
+# repos use prow.sh for that, but as csi-release-tools isn't a normal
+# repo with some Go code in it, it has a custom Prow test script.
+
+./verify-shellcheck.sh "$(pwd)"