Fix for multiple env vars in config parameters

[c04x]

trafficstars

Release notes

[rn:skip]

What does this PR do?

This fixes a bug #15896.

Why is it important/What is the impact to the user?

This PR now allows the user to combine env vars with hard coded strings and env vars in java plugins.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] ~~I have made corresponding changes to the documentation~~
• [ ] ~~I have made corresponding change to the default configuration files (and/or docker env variables)~~
• [x] I have added tests that prove my fix is effective or that my feature works

Related issues

• Closes #15896

[cla-checker-service[bot]]

💚 CLA has been signed