Because OpenShift deployment is done via immutable container it is not possible to deploy alerting action plugins in an ad-hoc way (as it typically the case). And also because the action plugins must be deployed alongside the core alerting war the required plugins need to be included in the metrics container. The simplest way to do this is likely to add the necessary plugin wars to the metrics ear, and therefore no additional deployments are needed.
IT may make sense to deploy the same plugins in the standalone and services component ears as well. I'll look into that.