Add GC of layers in Clair

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

util/secscan/api.py

@@ -28,6 +28,7 @@ class APIRequestFailure(Exception):
 
 _API_METHOD_INSERT = 'layers'
 _API_METHOD_GET_LAYER = 'layers/%s'
+_API_METHOD_DELETE_LAYER = 'layers/%s'
 _API_METHOD_MARK_NOTIFICATION_READ = 'notifications/%s'
 _API_METHOD_GET_NOTIFICATION = 'notifications/%s'
 _API_METHOD_PING = 'metrics'
@@ -122,6 +123,26 @@ class SecurityScannerAPI(object):
       'Layer': layer_request,
     }
 
+  def delete_layers(self, layers):
+    """ Deletes the given layers in the security scanner. """
+    if self._config is None:
+      # Not configured.
+      return
+
+    for layer in layers:
+      layer_id = '%s.%s' % (layer.docker_image_id, layer.storage.uuid)
+      self.delete_layer(layer_id)
+
+  def delete_layer(self, layer_id):
+    """ Calls DELETE on the layer with the given ID in the security scanner, removing it from
+        its database.
+    """
+    try:
+      response = self._call('DELETE', _API_METHOD_DELETE_LAYER % layer_id)
+      return response.status_code / 100 == 2
+    except requests.exceptions.RequestException:
+      logger.exception('Failed to delete layer: %s', layer_id)
+      return False
 
   def ping(self):
     """ Calls GET on the metrics endpoint of the security scanner to ensure it is running