At this point (as of KNIME Server 4.9.2), credentials should be passed via Workflow Credentials. To set this up, you need to add a set of credentials to the workflow -> right-click workflow in local repository -> Workflow Credentials. No need to save a password. In the WF, you set the DB Connector node to use those credentials. Then upload to KNIME Server. Once you run the job, either ad-hoc or on schedule, there will be a prompt asking for the credentials. Of course, the credentials you enter there can be for your generic user as well.
We’ll make this fair bit easier with the release of KNIME Server 4.10, scheduled for December release. There, we add an additional “Configuration” tab to the execution dialog, which grants you access to all configuration nodes in a workflow. All you’d need to do then is add a Credentials Configuration node to your WF, connected to the DB Connector via flow variable.
Hope that helps!
Edit: I need to add that the solution I mentioned for 4.9.2 will only work for EJB mountpoints. It will not work for REST mountpoints.