Expose scan template level control scanning options

* `ScanTemplate#control_scanning?`
* `ScanTemplate#control_scanning=`
I tend to shy away from adding checks like this to duplicate UI logic. What happens if it is enabled and you try to save? Will Nexpose throw or return an error? If it does and that message is clear enough, then this check only saves a small roundtrip and avoids the gem having to repeat UI validation.

On a separate note, I see "control" and "controls" in different places. What's the official term? We should use that where possible (if we aren't). My gut is "controls", but UX/Doc would be the final word.