Reduce default number of workers to 1

[pacoorozco]

What issue type does this pull request address? (keep at least one, remove the others)
/kind enhancement

What is this pull request for? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves #271

Does this pull request has user-facing changes? (e.g. config changes, new/modified commands, new/modified flags)
Change the default number of workers to 1. So it will not use concurrency by default.

[codecov]

Codecov Report

Merging #273 (9b45c19) into master (4702c0e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   63.87%   63.87%           
=======================================
  Files          21       21           
  Lines         609      609           
=======================================
  Hits          389      389           
  Misses        195      195           
  Partials       25       25           

Impacted Files	Coverage Δ
internal/cmd/push.go	13.33% <100.00%> (ø)
internal/config/config.go	84.61% <100.00%> (ø)

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