Fix the way content entity datasources save the selected bundles

Currently, the export of an index contains something like this:

datasource_configs:
  'entity:node':
    default: '1'
    bundles:
      article: '1'
      page: '0'
  'entity:taxonomy_term':
    default: '1'
    bundles:
      tags: '0'

But, of course, instead of the FAPI-internal representation of select field values, we should have just a sequence of the selected values:

datasource_configs:
  'entity:node':
    default: '1'
    bundles:
      - article
  'entity:taxonomy_term':
    default: '1'
    bundles: { }

Estimated Value and Story Points

This issue was identified as a Beta Blocker for Drupal 8. We sat down and figured out the value proposition and amount of work (story points) for this issue.

Value and Story points are in the scale of fibonacci. Our minimum is 1, our maximum is 21. The higher, the more value or work a certain issue has.

Value : 1
Story Points: 5