void validateBcp(Bcp bcp) {
        if (bcp.isEmpty()) return;
        if ( ! prodRegions().equals(bcp.regions()))
            throw new IllegalArgumentException("BCP and deployment mismatch in " + this + ": " +
                                               "A <bcp> element must place all deployed production regions in " +
                                               "at least one group, and declare no extra regions. " +
                                               "Deployed regions: " + prodRegions() +
                                               ". BCP regions: " + bcp.regions());
}
Validates the given BCP instance (which is owned by this, or if none, a default) against this instance.
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentInstanceSpec.java