Fix return value for index#list and index#mDelete

[Aschen]

What does this PR do?

This PR fix the return value for index#list and index#mDelete.
They now returns array of strings instead of an object containing only one property.

How should this be manually tested?

• Step 1 : Create some indexes for i in {1..10}; do; curl -H "Content-Type: application/json" -X POST "http://localhost:7512/test$i/_create" &; sleep 0.05; done
• Step 2 : List them await kuzzle.index.list() and check return value
• Step 3 : Delete them `await kuzzle.index.mDelete(['test1', 'test2']) and check return value

[codecov-io]

Codecov Report

Merging #320 into 6-dev will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           6-dev    #320   +/-   ##
=====================================
  Coverage   97.5%   97.5%           
=====================================
  Files         28      28           
  Lines       1400    1400           
=====================================
  Hits        1365    1365           
  Misses        35      35

Impacted Files	Coverage Δ
src/controllers/index.js	97.36% <100%> (ø)	⬆️

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[ScreamZ]

Nice one