feat: allow custom kwok instance-types at runtime

[alec-rabold]

Description

Implements #1848

Adds a new flag and environment variable to the KWOK cloud-provider to provide a way to use custom instance-types at runtime (currently requires recompiling). The main use-case is not having to maintain a unique image for each combination of instance-types.

Included some boiler-plate for options; can remove if preferred

How was this change tested?

Locally tested with default and custom instance-types

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[k8s-ci-robot]

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This pull-request has been approved by: alec-rabold
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[k8s-ci-robot]

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[coveralls]

Pull Request Test Coverage Report for Build 12045581115

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 80.863%

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Files with Coverage Reduction	New Missed Lines	%
pkg/test/expectations/expectations.go	2	94.73%

Totals
Change from base Build 12037226338:	0.02%
Covered Lines:	8903
Relevant Lines:	11010

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💛 - Coveralls