Repository: docker/docker-py
Issue: registry auth for docker service does not work on Windows

Credentials to authenticated docker registry are not passed to docker service on Windows after login:
```
client.login(username, password, registry=registry, reauth=True)
client.services.create([any_args])
```

On Centos7 it works fine.
Tested with AWS ECR.

Debug logs (red lines - Windows, green lines - Centos7):
```diff
[DEBUG]: Looking for auth entry for 'XXX.amazonaws.com'
[DEBUG]: No entry found
[DEBUG]: http://localhost:None "POST /v1.35/auth HTTP/1.1" 200 48
[DEBUG]: {u'IdentityToken': u'', u'Status': u'Login Succeeded'}
[DEBUG]: Looking for auth config
- [DEBUG]: Using credentials store "wincred"
[DEBUG]: Looking for auth entry for 'XXX.amazonaws.com'
- [DEBUG]: No entry found
- [DEBUG]: No auth config found
+ [DEBUG]: Found 'XXX.amazonaws.com'
+ [DEBUG]: Found auth config
[DEBUG]: http://localhost:None "POST /v1.35/services/create HTTP/1.1" 201 35
[DEBUG]: http://localhost:None "GET /v1.35/services/ixpf3gaiy0cep3qzj9ly302n6 HTTP/1.1" 200 None
```

Windows environment:
```
docker==2.7.0
docker-pycreds==0.2.1
Python 2.7.14
Client:
 Version:       17.12.0-ce
 API version:   1.35
 Go version:    go1.9.2
 Git commit:    c97c6d6
 Built: Wed Dec 27 20:05:22 2017
 OS/Arch:       windows/amd64

Server:
 Engine:
  Version:      17.12.0-ce
  API version:  1.35 (minimum version 1.12)
  Go version:   go1.9.2
  Git commit:   c97c6d6
  Built:        Wed Dec 27 20:12:29 2017
  OS/Arch:      linux/amd64
  Experimental: true
```

Centos7 environment:
```
docker==2.7.0
docker-pycreds==0.2.1
Python 2.7.5
Client:
 Version:       17.12.0-ce
 API version:   1.35
 Go version:    go1.9.2
 Git commit:    c97c6d6
 Built: Wed Dec 27 20:10:14 2017
 OS/Arch:       linux/amd64

Server:
 Engine:
  Version:      17.12.0-ce
  API version:  1.35 (minimum version 1.12)
  Go version:   go1.9.2
  Git commit:   c97c6d6
  Built:        Wed Dec 27 20:12:46 2017
  OS/Arch:      linux/amd64
  Experimental: false
```

Changed files:
tests/unit/auth_test.py, docker/auth.py

diff --git a/docker/auth.py b/docker/auth.py
index 91be2b85..48fcd8b5 100644
--- a/docker/auth.py
+++ b/docker/auth.py
@@ -90,9 +90,12 @@ def resolve_authconfig(authconfig, registry=None):
             log.debug(
                 'Using credentials store "{0}"'.format(store_name)
             )
-            return _resolve_authconfig_credstore(
+            cfg = _resolve_authconfig_credstore(
                 authconfig, registry, store_name
             )
+            if cfg is not None:
+                return cfg
+            log.debug('No entry in credstore - fetching from auth dict')
 
     # Default to the public index server
     registry = resolve_index_name(registry) if registry else INDEX_NAME
diff --git a/tests/unit/auth_test.py b/tests/unit/auth_test.py
index d6981cd9..ee32ca08 100644
--- a/tests/unit/auth_test.py
+++ b/tests/unit/auth_test.py
@@ -210,6 +210,19 @@ class ResolveAuthTest(unittest.TestCase):
             self.auth_config, auth.resolve_repository_name(image)[0]
         ) is None
 
+    def test_resolve_auth_with_empty_credstore_and_auth_dict(self):
+        auth_config = {
+            'auths': auth.parse_auth({
+                'https://index.docker.io/v1/': self.index_config,
+            }),
+            'credsStore': 'blackbox'
+        }
+        with mock.patch('docker.auth._resolve_authconfig_credstore') as m:
+            m.return_value = None
+            assert 'indexuser' == auth.resolve_authconfig(
+                auth_config, None
+            )['username']
+
 
 class CredStoreTest(unittest.TestCase):
     def test_get_credential_store(self):