Restore "latest" for Ubuntu images in test templates

[mboersma]

What type of PR is this?:

/kind cleanup

What this PR does / why we need it:

In #2618 we pinned some Ubuntu image references to the same Last Known Good version as Windows, but that version doesn't exist for the "capi" Linux offer. I think it should work to leave Ubuntu at "latest" since we have logic that negotiates agreement on a version between Windows and Linux. (Alternately, we can pin these to the correct Ubuntu version, published several days earlier.)

Which issue(s) this PR fixes:

(Bug reported in chat by @lzhecheng.)

Special notes for your reviewer:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[jackfrancis]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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[jackfrancis]

/cherry-pick release-1.4

[k8s-infra-cherrypick-robot]

@jackfrancis: once the present PR merges, I will cherry-pick it on top of release-1.4 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.4

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[k8s-infra-cherrypick-robot]

@jackfrancis: new pull request created: #2624

In response to this:

/cherry-pick release-1.4

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