In auto-config - rework to allow for single dropdown for agent selection

In the auto-config wizard, the agent/target system dropdown lists the agents. The simplest option was chosen to add a second dropdown to contain PowerShell, File Changes and None, which was visible if the agent was selected as _Custom (originally none).

Consider combining the two dropdowns.

This would currently take a lot of effort, as the dropdown is bound to agent id and has the potential to break, as such the fix version will be v3.2.