Configuring the datasources works the same in the cloud version as it does on the server one.
However, you know that on the server version you have to have the JDBC Driver already loaded (for instance placed in Jira's lib folder). Since you're in cloud, you cannot access that directory anymore so, the datasources page was updated so that now you can add the JDBC drivers directly from the UI.
In order to add a new JDBC Driver you have to press the "Choose File" button, select your driver and then press the "Upload" button. After doing this, the newly updated driver will appear on the page as well.
In order for the updated driver to be usable, you'll have to go to SIL Engine Mgmt page and restart the engine.
You can now use the datasources on cloud as well! 