Prepare kubectl and helm deployers for --kubeconfig flag

[corneliusweig]

Relates to #2699

Should merge before : #3100

Should merge after : n/a

Description

Adds the infrastructure to pass the --kubeconfig CLI setting to kubectl and helm.

User facing changes

n/a

Next PRs.

• Prepare cli-go to accept the --kubeconfig setting (Prepare cli-go to accept --kubeconfig setting #3107)
• Add a new CLI flag --kubeconfig to configure the kubeconfig (Add --kubeconfig CLI flag to all relevant commands #3100)

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

[codecov]

Codecov Report

Merging #3108 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/config/options.go	100% <ø> (ø)	⬆️
pkg/skaffold/kubectl/cli.go	100% <100%> (ø)	⬆️
pkg/skaffold/deploy/helm.go	77.5% <100%> (+0.24%)	⬆️