[stable/mongodb-replicaset]: use mongodb as port name to allow DNS seedlists

[andrenarchy]

What this PR does / why we need it:
By changing the service's port name to mongodb, Kubernetes automatically creates SRV DNS records under _mongodb._tcp.my-mongo-service.namespace.svc.cluster.local that point to the running instances (including the port for each instance). This is exactly what MongoDB clients support in the DNS Seedlist Connection Format. The benefit is that for replica sets you can use a connection URI like mongodb+srv://my-mongo-service.namespace.svc.cluster.local/?replicaSet=my-rs that has a single host name. Otherwise you have to construct a URI with all hostnames of the replica set which is a bit tedious.

Special notes for your reviewer:
This only looks like a cosmetic change but the name mongodb is expected by MongoDB clients, see https://docs.mongodb.com/manual/reference/connection-string/#dns-seedlist-connection-format. peer was arbitrary I assume so it shouldn't affect anything. I also tested it and it works fine.

ping @foxish @unguiculus

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: helpdesk@rt.linuxfoundation.org

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[andrenarchy]

@k8s-ci-robot /check-cla

[unguiculus]

Please bump the chart version

[andrenarchy]

@unguiculus done

[unguiculus]

/ok-to-test

[unguiculus]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andrenarchy, unguiculus

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• stable/mongodb-replicaset/OWNERS [unguiculus]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment