Fix DSS API documentation

[Bento007]

Instantiate DSSClient to generate the documentation for swagger API binding

[codecov-io]

Codecov Report

Merging #227 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #227      +/-   ##
=========================================
+ Coverage   83.39%   83.4%   +0.01%     
=========================================
  Files          37      37              
  Lines        1554    1555       +1     
=========================================
+ Hits         1296    1297       +1     
  Misses        258     258

Impacted Files	Coverage Δ
hca/dss/__init__.py	84.04% <100%> (+0.09%)	⬆️

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[kislyuk]

I think the way to fix this is not by introducing new import-time side effects, but to make sure instantiation of the client happens in docs/conf.py. I think the regression happened in the following diff, and just reverting its effect should fix this:

https://github.com/HumanCellAtlas/dcp-cli/pull/146/files#diff-85987f48f1258d9ee486e3191495582dL25

You might consider inserting a comment to not accidentally remove these imports again.

[kislyuk]

(Also, in re: #146, whenever changing the doc build process, a good practice is to diff the output of the old build vs. the new.)