Enable multiple kustomizations in the kustomize deployer

[corneliusweig]

Fixes #1872.

Description

So far, the kustomize deployer only accepted a single path to a kustomization folder. This required an umbrella kustomization when working with multiple kustomization folders. On the other hand, for helm it has always been possible to deploy multiple charts, so that the UX between the differnent deployers was kind of inconsistent as pointed out in #1872.
Therefore, this PR enables multiple kustomization folders/paths in a single deployer stanza.

User facing changes

The (string) key changed to deploy.kustomize.paths ([]string). When not specified, it defaults to []string{"."} and behaves equivalently to the previous config version.

Before

The pipeline had a key deploy.kustomize.path of type string to specify a single kustomization folder. When empty it defaults to ".".

After

The pipeline has a key deploy.kustomize.paths of type []string to specify a multiple kustomization folders. When empty (nil or length zero) it defaults to []string{"."}.

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.

Release Notes

Examples of user facing changes:
- The Kustomize deployer now handles multiple kustomization root paths.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1e6a52c). Click here to learn what that means.
The diff coverage is 92%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)
pkg/skaffold/schema/defaults/defaults.go	94.41% <100%> (ø)
pkg/skaffold/schema/v2alpha2/upgrade.go	85.71% <100%> (ø)
pkg/skaffold/deploy/deploy_mux.go	89.13% <100%> (ø)
pkg/skaffold/deploy/kustomize.go	71.62% <87.5%> (ø)

[balopat]

This looks good to me, thank you @corneliusweig! And thanks for fixing the init_test.go bug - our lines has crossed there somehow with out of order PR merges or something.