The right place to implement this would be in a new method of teuthology.orchestra.Cluster - maybe like this:
```
    def first(self, role):
        # Use this method if you don't care how many there are but you want the first
        do_stuff_here()

    def only(self, role):
        # Use this method if you want the test to fail if there are more than one
        do_stuff_here()
```