Fix SSH issue with in place upgrades.

[k4leung4]

What this PR does / why we need it:
The master in-place upgrade is currently broken due to an SSH failure
with the error "Host key verification failed."
As the master machine may change, it should be safe to disable the host
key check, which fixes the issue.

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 #182

Special notes for your reviewer:

Release note:

@kubernetes/kube-deploy-reviewers

[k8s-ci-robot]

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[k4leung4]

/assign @karan @jessicaochen

[karan]

did you try UserKnownHostsFile=/dev/null along with what you already have?

[k4leung4]

added that option to ssh as well.

[k4leung4]

/unassign @jessicaochen

/assign @medinatiger

[k4leung4]

Verified in-place upgrades works.

[karan]

Does that mean the control plane works after the upgrade, or only that the image was upgraded?

[k4leung4]

Both, was able to upgrade control plane from 1.9.4 to 1.9.7, and the master is responsive after the upgrade, verified by running kubectl get machines

[karan]

Sweet.

/lgtm