Config: Make neo4j database configurable

[gschuurman]

Allow the neo4j database to be configurable, this is required when not in charge of the neo4j deployment.

Checklist

• [x] The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• [ ] Links to related issues (if applicable)
• [x] Tests for the changes have been added/updated (if applicable)
• [x] Docs related to the changes have been added/updated (if applicable)