Drops celery_taskmeta collection

re #1084
https://pulp.plan.io/issues/1084

server/pulp/server/db/migrations/0020_drop_celery_taskmeta.py

@@ -0,0 +1,18 @@
+"""
+This migration removes the `celery_taskmeta` collection.
+"""
+from pulp.server.db import connection
+
+
+def migrate(*args, **kwargs):
+    """
+    Perform the migration as described in this module's docblock.
+
+    :param args:   unused
+    :type  args:   list
+    :param kwargs: unused
+    :type  kwargs: dict
+    """
+    db = connection.get_database()
+    collection = db['celery_taskmeta']
+    collection.drop()

server/test/unit/server/db/migrations/test_0020_drop_celery_taskmeta.py

@@ -0,0 +1,23 @@
+"""
+This module contains tests for pulp.server.db.migrations.0020_drop_celery_taskmeta.py
+"""
+import unittest
+
+from mock import patch
+
+from pulp.server.db.migrate.models import _import_all_the_way
+
+migration = _import_all_the_way('pulp.server.db.migrations.0020_drop_celery_taskmeta')
+
+
+class TestMigrate(unittest.TestCase):
+    """
+    Test the migrate() function.
+    """
+
+    @patch.object(migration.connection, 'get_database')
+    def test_celery_taskmeta_collection_dropped(self, mock_get_database):
+        mock_get_database.return_value.collection_names.return_value = []
+        collection = mock_get_database.return_value['celery_taskmeta']
+        migration.migrate()
+        collection.drop.assert_called_once()