tests: test_pull.py doesn't fetch fedora each time

Use the image already fetched through Docker of the fedora image instead
of fetching it over the network each time.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #503
Approved by: cgwalters

tests/unit/test_pull.py

@@ -5,7 +5,7 @@ from Atomic import atomic
 class TestAtomicPull(unittest.TestCase):
     class Args():
         def __init__(self):
-            self.image = "fedora"
+            self.image = "docker:fedora"
             self.user = False
 
     def test_pull_as_privileged_user(self):