Add build_root image configuration for prow

ci-operator is the tool that handles the process of
building images and running workflows in Prow jobs.
By adding this configuration in the repo, we can replace
the base root image (which will serve as base for most of the
jobs) along with code changes, when upgrading dependencies (golang
for instance)
This file has no effect for now, only after updating ci-operator
config on openshift/release, in another PR.

.ci-operator.yaml

@@ -0,0 +1,4 @@
+build_root_image:
+  name: tools
+  namespace: openstack-k8s-operators
+  tag: ci-build-root-golang-1.19-sdk-1.26