fix: protect the slice for append operation in multi-thread scenario

[andyliuliming]

for more details about the issue please take a look at this:
https://medium.com/@cep21/gos-append-is-not-always-thread-safe-a3034db7975

[andyliuliming]

@thomas1206

[andyliuliming]

Hi @mboersma , I think there's some issue on the circleci golang lint.
it ask us to do these changes for the style:
"github.com/mattn/go-colorable" => colorable "github.com/mattn/go-colorable"

and all the api version model:
"github.com/Azure/aks-engine/pkg/api/v20160330" =>v20160330 "github.com/Azure/aks-engine/pkg/api/v20160330"

will leave it to you whether we need changes like that..

[zhongyi-zhang]

Agree. This is something we should take care of. The underlying issue could be hard to track and repro.

[andyliuliming]

/assign @mboersma

[mboersma]

Thanks @andyliuliming. We may have a fix for the goimports check in #204. I'll get that fixed and look at your PR today.

[tariq1890]

@andyliuliming Could you rebase this PR?

[andyliuliming]

@andyliuliming Could you rebase this PR?

rebased, thanks :)

[tariq1890]

Approved with minor comments.

[andyliuliming]

@mboersma could you please help merge? Thanks.

[tariq1890]

@andyliuliming can you address my last comment? I can merge after that

[codecov]

Codecov Report

Merging #198 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #198      +/-   ##
=========================================
+ Coverage   53.18%   53.2%   +0.02%     
=========================================
  Files          95      95              
  Lines       14228   14235       +7     
=========================================
+ Hits         7567    7574       +7     
  Misses       5995    5995              
  Partials      666     666

[codecov]

Codecov Report

Merging #198 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #198      +/-   ##
=========================================
+ Coverage   53.18%   53.2%   +0.02%     
=========================================
  Files          95      95              
  Lines       14228   14235       +7     
=========================================
+ Hits         7567    7574       +7     
  Misses       5995    5995              
  Partials      666     666

[andyliuliming]

@andyliuliming can you address my last comment? I can merge after that

done, thanks very much :)

[tariq1890]

/lgtm

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