Make defaultMapResource function public to allow reusing it in custom functions (trying to fix Travis CI)

[nilebox]

In case there is a need to handle extra resource types, it's useful to be able to keep the original behavior, e.g.

func myMapResource(res *resource.Resource) *monitoredrespb.MonitoredResource {
  if res.Type == "my-resource-type" {
    // custom handling
    ...
    return result
  }

  // otherwise return default result
  return stackdriver.DefaultMapResource(res)
}

[codecov-io]

Codecov Report

Merging #256 into master will not change coverage by %. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #256   +/-   ##
=======================================
  Coverage   72.02%   72.02%           
=======================================
  Files          17       17           
  Lines        1691     1691           
=======================================
  Hits         1218     1218           
  Misses        397      397           
  Partials       76       76           

Impacted Files	Coverage Δ
resource.go	100.00% <100.00%> (ø)
stackdriver.go	31.68% <100.00%> (ø)

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