Use a first_found lookup instead of a with_first_found loop so that
the 'paths' parameter can be used.
This ensures that only vars from the role are included, and not vars
from a parent calling role. This can happen when a parent role has
a higher priority vars file available for inclusion than the role
it calls.
Change-Id: I6b68d4d15ae516d23c88b3c4c21a076e8d54604e