Allow deeply nested property definition for Helm properties

[gsquared94]

Fixes: #500
Description

Being unable to define verbose nested properties in certain helm sections of the skaffold yaml file has been a cause of user confusion since it is inconsistent with other sections that allow it. Several evidences in issue #500

This PR allows defining helm properties like:

artifactOverrides:
    image: skaffold-helm
    skaffold-helm-subchart:
        image: skaffold-helm
overrides:
   some:
       key: someValue
setValues:
   some:
       key: someValue

which makes it consistent across artifactOverrides, overrides and setValues.

The old way would have been:

artifactOverrides:
    image: skaffold-helm
    skaffold-helm-subchart.image: skaffold-helm
overrides:
   some:
       key: someValue
setValues:
   some.key: someValue

The change is backward compatible so it doesn't need any config update function.

[codecov]

Codecov Report

Merging #4511 into master will decrease coverage by 0.01%.
The diff coverage is 70.37%.

@@            Coverage Diff             @@
##           master    #4511      +/-   ##
==========================================
- Coverage   72.37%   72.36%   -0.02%     
==========================================
  Files         333      333              
  Lines       12959    12983      +24     
==========================================
+ Hits         9379     9395      +16     
- Misses       2983     2988       +5     
- Partials      597      600       +3     

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100.00% <ø> (ø)
pkg/skaffold/schema/util/util.go	76.66% <70.37%> (-5.16%)	⬇️
pkg/skaffold/update/update.go	53.57% <0.00%> (-5.81%)	⬇️
cmd/skaffold/app/cmd/flags.go	86.36% <0.00%> (ø)
cmd/skaffold/app/cmd/cmd.go	66.42% <0.00%> (+0.24%)	⬆️

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[nkubala]

one nit but this LGTM, tried it out and it works nicely