Fix Glacier test failure in python 3.5 due to MagicMock

In Python 3.5+, the ConcurrentUploader tests were failing due to an
equality check.

```
FAIL: test_correct_low_level_api_calls (tests.unit.glacier.test_concurrent.TestConcurrentUploader)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/boto/tests/unit/glacier/test_concurrent.py", line 105, in test_correct_low_level_api_calls
    'vault_name', mock.ANY, mock.ANY, 8 * 1024 * 1024)
  File "/home/ryansb/.pyenv/versions/3.5.0/lib/python3.5/unittest/mock.py", line 792, in assert_called_with
    raise AssertionError(_error_message()) from cause
AssertionError: Expected call: complete_multipart_upload('vault_name', <ANY>, <ANY>, 8388608)
Actual call: complete_multipart_upload('vault_name', <MagicMock name='mock.initiate_multipart_upload().__getitem__()' id='140364396570440'>, b'c5aac592460a9ac
7845e341090f6f9c81f201b63e5338ee8948a6fe6830c55dc', 8388608)
```

Note that the MagicMock in the second call position was failing to be equal to mock.ANY.

In a Python 3.5 shell, this can be demonstrated as:

```
>>> from unittest import mock
>>> m = mock.MagicMock()
>>> mock.ANY == m
True
>>> m == mock.ANY
False
```

The tests pass in all supported versions below 3.4, and to get around
this in 3.5 I added a return value for the `initiate_multipart_upload`
attribute of the mock that contains a request ID to be checked. This
prevents `complete_multipart_upload` from being called with the
MagicMock.

tests/unit/glacier/test_concurrent.py

@@ -95,14 +95,20 @@ def test_work_queue_is_correctly_populated(self):
 
     def test_correct_low_level_api_calls(self):
         api_mock = mock.MagicMock()
+        upload_id = '0898d645-ea45-4548-9a67-578f507ead49'
+        initiate_upload_mock = mock.Mock(
+            return_value={'UploadId': upload_id})
+        # initiate_multipart_upload must return a body containing an `UploadId`
+        api_mock.attach_mock(initiate_upload_mock, 'initiate_multipart_upload')
+
         uploader = FakeThreadedConcurrentUploader(api_mock, 'vault_name')
         uploader.upload('foofile')
         # The threads call the upload_part, so we're just verifying the
         # initiate/complete multipart API calls.
-        api_mock.initiate_multipart_upload.assert_called_with(
+        initiate_upload_mock.assert_called_with(
             'vault_name', 4 * 1024 * 1024, None)
         api_mock.complete_multipart_upload.assert_called_with(
-            'vault_name', mock.ANY, mock.ANY, 8 * 1024 * 1024)
+            'vault_name', upload_id, mock.ANY, 8 * 1024 * 1024)
 
     def test_downloader_work_queue_is_correctly_populated(self):
         job = mock.MagicMock()