When checking for the existence of task files at a given path, the path
should be prefaced with {{playbook_dir}} so that the correct absolute
path is checked. Otherwise, the path is relative to the working
directory which does not have to be the same as playbook_dir.
Change-Id: Ia16f249b240c06128374551f6110419f9c1923c9
Signed-off-by: James Slagle <jslagle@redhat.com>