[BUG] Set default data source unstable issue

[zhyuanqi]

Describe the bug
The first created datasource should be automatically set as the default datasource. However, this feature is unstable. Sometimes the first created datasource has not been selected as default datasource. It is flaky

To Reproduce
Steps to reproduce the behavior:

1. Go to management page
2. Click on Datasources ---> create datasource
3. Create datasource and save datasource
4. Datasource has been saved, but it has not been selected as default datasource

Expected behavior
The first created datasource should be automatically set as the default datasource.

OpenSearch Version
OS_2.14

Dashboards Version
OSD_2.14

[BionIT]

@zhyuanqi @yujin-emma Is the tag correct? We are targeting to fix in 2.15, right?