enable to configure luis publishing slot enviroment

[feich-ms]

fix #855 to enable users to configure the publishing slot enviroment. Before this change, lu build will default publish to production slot with no configuration capability. After the fix, users can specify --isStaging in lu:build to publish to staging slot. Default to production slot if not specified.

[boydc2014]

I know the issue is described in #855, but it would be good to summarize what this PR actually did for better review transpancy

[codecov-commenter]

Codecov Report

Merging #898 into master will decrease coverage by 0.22%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #898      +/-   ##
==========================================
- Coverage   57.94%   57.72%   -0.23%     
==========================================
  Files         218      217       -1     
  Lines       15365    15150     -215     
  Branches     2146     2119      -27     
==========================================
- Hits         8904     8745     -159     
+ Misses       5879     5844      -35     
+ Partials      582      561      -21     

Impacted Files	Coverage Δ
packages/lu/src/parser/lubuild/builder.ts	12.80% <0.00%> (ø)
packages/lu/src/parser/lubuild/core.ts	16.29% <ø> (ø)
packages/qnamaker/src/commands/qnamaker/build.ts	76.84% <ø> (ø)
packages/luis/src/commands/luis/build.ts	80.00% <100.00%> (ø)
packages/lg/src/commands/lg/index.ts	66.66% <0.00%> (-16.67%)	⬇️
packages/luis/src/commands/luis/index.ts	66.66% <0.00%> (-16.67%)	⬇️
...kages/luis/src/commands/luis/application/delete.ts	71.42% <0.00%> (-4.58%)	⬇️
...kages/luis/src/commands/luis/application/import.ts	53.84% <0.00%> (-1.06%)	⬇️
packages/luis/src/api/http-request.ts	86.66% <0.00%> (-0.84%)	⬇️
... and 33 more

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