Add option in global config to define default kube-contexts

[corneliusweig]

See design proposal #2384
Depends on #2447, #2509, and #2555 (all prerequisites have been merged)

[codecov]

Codecov Report

Merging #2558 into master will increase coverage by 0.13%.
The diff coverage is 83.5%.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/cmd.go	68% <ø> (-0.26%)	⬇️
cmd/skaffold/app/cmd/config.go	100% <100%> (ø)	⬆️
cmd/skaffold/app/cmd/config/flags.go	100% <100%> (ø)	⬆️
cmd/skaffold/app/cmd/runner.go	67.44% <63.63%> (-1.79%)	⬇️
cmd/skaffold/app/cmd/config/set_context.go	75.6% <75.6%> (ø)
pkg/skaffold/config/util.go	65.55% <81.81%> (+2.26%)	⬆️
pkg/skaffold/schema/profiles.go	88.07% <0%> (-2.4%)	⬇️
pkg/skaffold/server/server.go	58.57% <0%> (ø)	⬆️
pkg/skaffold/util/config.go	78.94% <0%> (+10.52%)	⬆️

[corneliusweig]

As discussed in #2510 (comment), the global config option for the kube-context might be an overengineering of the kube-context configurability.

Therefore, I'm going to close this for now to put these changes on hold. If somebody sees an urgent need to get this functionality in, I'm happy to revive this PR. But let's first see how users get along with just the CLI option and the skaffold.yaml option. For now, I'll adjust the design proposal accordingly.

At the very least #2325 is affected.
CC @braderhart, @seblegall @woodcockjosh as you were involved in the discussion there.