image.tag does not return anything
There's no return statement in `images.tag`:
https://github.com/docker/docker-py/blob/master/docker/models/images.py#L99

[Readthedocs](https://docker-py.readthedocs.io/en/stable/images.html) (and the method comments) suggest it should return a bool for success.

I saw this running version 2.2.1 of the library
```
# pip freeze | grep docker
docker==2.2.1
docker-pycreds==0.2.1
```

**Repro code:**
```
import docker
def test_tag(id):
    client = docker.DockerClient()
    image = client.images.get(id)
    tag_result = image.tag('test_image', tag='test_tag')
    if tag_result is None:
        print('oops')
```
diff --git a/docker/models/images.py b/docker/models/images.py
index a9ed65ee..e8af101d 100644
--- a/docker/models/images.py
+++ b/docker/models/images.py
@@ -96,7 +96,7 @@ class Image(Model):
         Returns:
             (bool): ``True`` if successful
         """
-        self.client.api.tag(self.id, repository, tag=tag, **kwargs)
+        return self.client.api.tag(self.id, repository, tag=tag, **kwargs)
 
 
 class ImageCollection(Collection):
diff --git a/tests/integration/models_images_test.py b/tests/integration/models_images_test.py
index 6d61e497..881df0a1 100644
--- a/tests/integration/models_images_test.py
+++ b/tests/integration/models_images_test.py
@@ -71,7 +71,8 @@ class ImageTest(BaseIntegrationTest):
         client = docker.from_env(version=TEST_API_VERSION)
         image = client.images.pull('alpine:latest')
 
-        image.tag(repo, tag)
+        result = image.tag(repo, tag)
+        assert result is True
         self.tmp_imgs.append(identifier)
         assert image.id in get_ids(client.images.list(repo))
         assert image.id in get_ids(client.images.list(identifier))
['tests/integration/models_images_test.py', 'docker/models/images.py']