Update README with new OVA links

[codenrhoden]

What this PR does / why we need it:
Updates README to point to updated OVA's for CentOS 7, Ubuntu 18.04, Ubuntu 20.04, and Photon 3. This is the first time links to Ubuntu 20.04 images have been published

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1511

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

[srm09]

Hey @codenrhoden thanks for taking the time to create the newer set of images. I was hoping if there is a way for us to push newer images on a regular cadence via an automated pipeline?
I would love to hear your thoughts on this one. This effort might also help us to run the e2e testing suite against the latest k8s version.

[codenrhoden]

@srm09 Yeah, that's why I pinged @kkeshavamurthy on #1511. It's definitely something you two should talk about. :)

I think the pieces are in place if you wanted it be driven by Prow. I know CAPG does something similar -- it runs a nightly job to update the CAPG images if needed, and they run their CI against those new images daily (I think).

[MaxRink]

/lgtm
from my side
regarding CI, we have tome more advanced test cases we use for images (like not only bootstrapping but also trying out things like CSI (which depend on host packages) and we would be open in contributing that work upstream :)

[srm09]

@MaxRink Can you create an issue for that part and we could start a discussion there?
@codenrhoden Thanks for publishing the updated images. I will talk to @kkeshavamurthy more for that part.

[srm09]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: srm09

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:

• OWNERS [srm09]

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