Allow to overwrite `real_system_username` per destination

[bernt-matthias]

This will allow to overwrite real_system_username per destination.

There are two places in the code left where still the global config is used:

• https://github.com/galaxyproject/galaxy/blob/dev/lib/galaxy/tools/imp_exp/init.py
• https://github.com/galaxyproject/galaxy/blob/9e01e37bc19abe0fc6ec91497bb5e7c6a9c83daa/lib/galaxy/tools/actions/upload_common.py

For my use case this is not relevant (I need to overwrite it for the distination where ITs are sent to).

How to test the changes?

(Select all options that apply)

• [ ] I've included appropriate automated tests.
• [ ] This is a refactoring of components with existing test coverage.
• [ ] Instructions for manual testing are as follows:
  • I tested it locally

License

• [x] I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.