Revert "Remove service identity as custom property in server config for Hikari onboarding"

[rakhiagr]

Reverts linkedin/datahub-gma#566

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 64.62%. Comparing base (34e0ee0) to head (ceb5ebe).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #574   +/-   ##
=========================================
  Coverage     64.61%   64.62%           
  Complexity     1662     1662           
=========================================
  Files           141      141           
  Lines          6563     6561    -2     
  Branches        803      802    -1     
=========================================
- Hits           4241     4240    -1     
  Misses         1977     1977           
+ Partials        345      344    -1     

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