skaffold render --output takes GCS file path

[ChrisGe4]

Fixes: #3902

Description

This PR adds gsutil.go that calls 'gsutil' to copy rendered manifests to a GCS bucket when skaffold render --output takes a file path starts with gs://

[codecov]

Codecov Report

Merging #3979 into master will decrease coverage by 0.04%.
The diff coverage is 61.29%.

Impacted Files	Coverage Δ
pkg/skaffold/deploy/util.go	65.62% <33.33%> (-13.55%)	⬇️
pkg/skaffold/deploy/deploy_mux.go	89.13% <100.00%> (ø)
pkg/skaffold/deploy/kubectl.go	66.40% <100.00%> (ø)
pkg/skaffold/util/gsutil.go	100.00% <100.00%> (ø)

[dgageot]

Hi @ChrisGe4, I'm a bit reluctant in merging such feature that avoids piping into gsutil. Can you tell us more about why you need this feature?

[ChrisGe4]

Hi @dgageot, we are interested in using Skaffold in a new project. One step in the workflow is uploading the hydrated manifests to a GCS bucket.
I choose to use gsutil because I found skaffold is directly calling kubectl so I used the same approach. I also had a meeting with several skaffold team members earlier and they had no objection to install gsutil in the skaffold cloud builder in the future. Those are the reasons why I created this PR.

[dgageot]

@ChrisGe4 ok. I'm still a bit reluctant since it looks like we could just pipe the output of skaffold to gsutil. But let's move on! I added a few comments.

[dgageot]

LGTM with nit

[nkubala]

LGTM also with two tiny nits

[nkubala]

@ChrisGe4 looks like you actually have one small linter error, can you check that out? not sure what happened with the kokoro job but it looks like it got stuck so I restarted it. when CI is green I'll merge