Clean up differences in bulkdata.xml examples

[punktilious]

Describe the bug The user guide mentions replacing the configDropins/defaults/bulkdata.xml with an example from configDropins/disabled/postgres or configDropins/disabled/derby but these are not equivalent.

Environment Which version of IBM FHIR Server? 5.0-RC2

Expected behavior Refactor so that the core bulkdata configuration is separate from the declaration of the datasource. Rename the examples to datasource-bulkdata.xml and update the documentation accordingly.