Update openstack json event schema definition

[eiffel777]

Update the json schema for openstack events to allow null for the user_name field. This allows events whose user_name field has a null value to be ingested. Any event with a user_name of null will be assigned to the unknown value when aggregated.

Tests performed

Tested in docker

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[plessbd]

Do we have any use cases in docker that this was actually hitting?
I didnt think we had any null cases in docker/

[jpwhite4]

Please can you briefly list the testing performed on this change.

[eiffel777]

@plessbd @jpwhite4 There weren't any cases for data that was already in the docker. I tested by spinning up a docker instance and then ingested some of the lakeeffect openstack data that did have null values in the user_name field. Before the change there were warnings such as this one.

2020-03-12 06:35:16 [warning] Record 8: NULL value found, but a string is required

These warning no longer occur once this change is applied.