Allow connecting from the Notification Controller to the Repo Server without TLS

[m-wynn]

Summary

All of the Argo CD components I have worked with have a way to disable TLS between each other via the argocd-cmd-params-cm ConfigMap. One issue I've run into is, when calling .repo.GetCommitMetadata in notifications templates, it tries to reach out to the Repo Server over TLS. There does not seem to be an easy way to disable it, short of overlaying the notifications template deployment itself and adding a command line flag.

Motivation

This is necessary for environments where Istio is managing mutual TLS between each component (#2784).

Proposal

Similar to the other controllers, we can add notificationscontroller.repo.server.plaintext to the argocd-cmd-params-cm ConfigMap, and then in the deployment template, map that to the ARGOCD_NOTIFICATION_CONTROLLER_REPO_SERVER_PLAINTEXT environment variable. In the go code for the notifications controller, we can attach this to &argocdRepoServerPlaintext. I'm proposing a singular "NOTIFICATION" here to match the existing ARGOCD_NOTIFICATION_CONTROLLER_SELF_SERVICE_ENABLED, but happy to change it.

It should be a small change so I'm happy to make a PR.