Hardcoded OS image/default username

### What happened
The Dashboard shows an incorrect username in the Node details. This information comes from a hardcoded map in the source code and does not work well with custom-built OS images not using the default cloud image usernames.

https://github.com/kubermatic/dashboard/blob/5656123bbf90e29ee5c4f178a30c7b0f27b95327/modules/api/pkg/machine/util.go#L26

### Expected behavior
One of the following options could solve the issue:

- When defining the default image in the Datacenter, allow specifying a pair of image + username instead. Ideally specifying a list of image + username pairs should be possible instead of entering a single image only.
- Introduce a configuration setting allowing to disable the visualization of the username in the Dashboard - either globally, or on project level to avoid confusing the users if non-standard OS images are used.

### How to reproduce
See screenshot above.

### Environment
- UI Version: 2.27.7
- API Version:
- Domain:
- Others:

### Current workaround
None

### Affected user persona
End-user

### Business goal to be improved
N/A

### Metric to be improved
N/A

Start with modules/api/pkg/machine/util.go at the linked hardcoded map, then trace how the value reaches the Dashboard's Node details. Review the two proposed configuration approaches and their impact on Datacenter defaults or username visibility. Done means custom-built OS images no longer display an incorrect default username.

Written by the indexing model from the issue text.