Simplify caching

[priyawadhwa]

This PR simplifies caching to the following rules, described in my
second design doc for artifact caching:

If remote cluster or push:true :

1. Check if image exists remotely. If it does, return.
2. Check if image exists locally. If it does, push.
3. If not, check if same digest exists locally. If it does, retag and push.
4. If not, rebuild (via standard build step).

If local cluster, and push:false:

1. Check if image exists locally. If it does, return.
2. If not, check if same digest exists locally. If it does, retag.
3. If not, rebuild (via standard build step).

[codecov]

Codecov Report

Merging #2452 into master will decrease coverage by 0.19%.
The diff coverage is 82.08%.

Impacted Files	Coverage Δ
pkg/skaffold/build/cache/retrieve.go	77.11% <82.08%> (-9.71%)	⬇️
pkg/skaffold/docker/remote.go	31.25% <0%> (-4.17%)	⬇️