Extended Kaniko support

[ricardo-larosa]

Fixes: #4852
Related: #4853

Description

Implement kaniko flags as skaffold config items.

User facing changes (remove if N/A)
Support for additionalFlags aka

        flags: 

is removed

Example:
Before

    artifacts:
    - image: registry.io/image:tag
      kaniko:
        flags: 
          - --snapshotMode=redo
          - --cleanup=true
          - --verbosity=debug

Now

    artifacts:
    - image: registry.io/image:tag
      kaniko:
        snapshotMode: redo
        cleanup: true
        verbosity: debug

Limitations/Scope

Does not implement kaniko --git tag nor related.

Notes
It looks long but is due to the number of flags implemented.

[codecov]

Codecov Report

Merging #4900 into master will increase coverage by 0.10%.
The diff coverage is 92.37%.

@@            Coverage Diff             @@
##           master    #4900      +/-   ##
==========================================
+ Coverage   71.96%   72.07%   +0.10%     
==========================================
  Files         357      358       +1     
  Lines       12327    12364      +37     
==========================================
+ Hits         8871     8911      +40     
+ Misses       2799     2797       -2     
+ Partials      657      656       -1     

Impacted Files	Coverage Δ
pkg/skaffold/build/cluster/kaniko.go	0.00% <0.00%> (ø)
pkg/skaffold/build/cluster/logs.go	15.78% <0.00%> (ø)
pkg/skaffold/schema/latest/config.go	100.00% <ø> (ø)
pkg/skaffold/build/cluster/pod.go	86.56% <80.00%> (-1.01%)	⬇️
pkg/skaffold/build/kaniko/args.go	95.23% <95.23%> (ø)
pkg/skaffold/build/cluster/secret.go	40.32% <100.00%> (ø)
pkg/skaffold/build/gcb/kaniko.go	77.77% <100.00%> (-4.58%)	⬇️
pkg/skaffold/schema/defaults/defaults.go	89.09% <100.00%> (ø)
pkg/skaffold/util/env_template.go	100.00% <100.00%> (ø)

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[tejal29]

Thanks for moving this into kaniko package!

[vinceferro]

Btw great stuff! I've been running hacks to have kaniko configured the way we wanted, now it's supported and I couldn't be happier :D