Kaniko improvements

[dgageot]

• Add more logs to better debug when it goes weird
• Auto detect the bucket name from the image names if no bucket is provided
• Let the user override the kaniko image
• Upgrade kaniko to 0.4.0
• Simplify the code

[codecov-io]

Codecov Report

Merging #1101 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1101   +/-   ##
=======================================
  Coverage   43.16%   43.16%           
=======================================
  Files          74       74           
  Lines        3408     3408           
=======================================
  Hits         1471     1471           
  Misses       1799     1799           
  Partials      138      138

Impacted Files	Coverage Δ
pkg/skaffold/docker/context.go	31.03% <0%> (ø)	⬆️

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