🐛 Accept empty string SSHKeyName in AWSCluster & AWSMachine validation webhooks

[dlipovetsky]

What type of PR is this?

/kind bug

What this PR does / why we need it:
Makes the AWSCluster & AWSMachine validation webhooks accept an empty string for SSHKeyName. That value is supported by the API.

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 #2306

Special notes for your reviewer:

• The webhook test files have slightly different sets of SSHKeyName validation tests. The SSHKeyName field is common to both AWSCluster and AWSMachine, so I think there should be one set of tests for the field. I can do this in a follow-up PR. WDYT?
• The first two commits add tests; they are expected to fail. The third commit adds the fix.

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

Fix the AWSCluster & AWSMachine validation webhooks, so that they accept an empty string for the `SSHKeyName` field.

[randomvariable]

/lgtm
/approve

[randomvariable]

thanks!

[k8s-ci-robot]

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[dlipovetsky]

api/v1alpha3/webhooks.go:35: File is not `gofmt`-ed with `-s` (gofmt)

Sorry, a lint (gofmt) issue. Will fix and force-push. Ironically, it's complaining about a missing newline at the end of the file. Meanwhile, the same commit fixed the imports 😂

[dlipovetsky]

@randomvariable Thanks for the review! I hope this makes it into the next release 😄

[sedefsavas]

/lgtm

Also @dlipovetsky +1 to have a single set of tests.

[dlipovetsky]

Thanks for the review @sedefsavas!