Apply resource labels in the deployer

[corneliusweig]

Relates to #2875, #3292

Should merge before : #3392

Description

This prepares for having multiple deployers per skaffold.yaml, and
ensures that every resource has the label of the proper deployer it was
deployed with.

User facing changes

n/a

Next PRs.

• add a deployer implementation that handles multiple deployers (i.e. kubectl, kustomize, helm)
• adapt skaffold.yaml to accept multiple deployer configurations in the deploy field.

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 #3390 into master will increase coverage by <.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/deploy/labels.go	9.75% <100%> (-6.1%)	⬇️
pkg/skaffold/runner/new.go	69.28% <100%> (ø)	⬆️
pkg/skaffold/deploy/kubectl.go	63.7% <100%> (ø)	⬆️
pkg/skaffold/deploy/helm.go	78.41% <100%> (+1.07%)	⬆️
pkg/skaffold/deploy/kustomize.go	76.61% <100%> (+2.41%)	⬆️