Improve port-forwarding

[dgageot]

Fixes #1360

[codecov-io]

Codecov Report

Merging #1452 into master will increase coverage by 0.12%.
The diff coverage is 44.82%.

@@            Coverage Diff             @@
##           master    #1452      +/-   ##
==========================================
+ Coverage   44.33%   44.46%   +0.12%     
==========================================
  Files         113      112       -1     
  Lines        4660     4642      -18     
==========================================
- Hits         2066     2064       -2     
+ Misses       2386     2371      -15     
+ Partials      208      207       -1

Impacted Files	Coverage Δ
pkg/skaffold/runner/dev.go	56.52% <100%> (+3.58%)	⬆️
pkg/skaffold/kubernetes/port_forward.go	40.96% <38.46%> (-0.53%)	⬇️

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