Admin should be able to turn shared runners into specific ones:

## What does this MR do?

Make sure admins could turn shared runners into specific runners.

## Are there points in the code the reviewer needs to double check?

Is this the desired behaviour?

## Why was this MR needed?

Closes #19039
Closes #19272

![Screen_Shot_2016-06-30_at_9.30.05_PM](/uploads/97eb3b4923fd4e498b1f8ca70b1345c8/Screen_Shot_2016-06-30_at_9.30.05_PM.png)

See merge request !4961
(cherry picked from commit b569f842)