Add GC of layers in Clair

Fixes https://www.pivotaltracker.com/story/show/135583207

test/test_secscan.py

@@ -547,6 +547,19 @@ class TestSecurityScanner(unittest.TestCase):
     notification = model.notification.create_repo_notification(repo, 'vulnerability_found', 'quay_notification', {}, {'level': 0})
     self.assertFalse(VulnerabilityFoundEvent().should_perform(event_data, notification))
 
+  def test_garbage_collection(self):
+    requests_made = set()
+
+    @urlmatch(netloc=r'(.*\.)?mockclairservice', path=r'/v1/layers/(.+)')
+    def delete_layer(url, request):
+      requests_made.add(request.method)
+
+    with HTTMock(delete_layer):
+      # Purge a repository.
+      model.repository.purge_repository('devtable', 'simple')
+
+    # Ensure that DELETE is called (and only DELETE) for the layers in the security scanner.
+    self.assertEquals(set(['DELETE']), requests_made)
 
   def test_notification_worker(self):
     pages_called = []