Naming restriction for template in configurations

[ecpullen]

From #145: "I wonder if we want to make this regex more strict on what we can match for the resource name or field name. Currently something weird like ${@@@@.!!!!!!} is accepted. Do we want to allow that? Maybe go with the kubernetes object name restrictions: https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names?"