Handle missing docker-py better (#27540)

* Update docker inventory to use APIClient

docker-py has been updated, and the `Client` class no longer
exists. We use the new `APIClient` class.

To provide graceful failure when docker-py is not installed,
we need to create a dummy `Client` class so that the inventory
script will get as far as displaying a useful error message

Before

```
$ contrib/inventory/docker.py --pretty
Traceback (most recent call last):
  File "contrib/inventory/docker.py", line 418, in <module>
    class AnsibleDockerClient(Client):
NameError: name 'Client' is not defined
```

After

```
$ contrib/inventory/docker.py --pretty
Failed to import docker-py. Try `pip install docker-py` - cannot import name Client
```

* docker inventory configuration file location

Allow docker.yml to live next to docker.py, as well as in the
current directory

contrib/inventory/docker.py

@@ -371,7 +371,6 @@ HAS_DOCKER_PY = True
 HAS_DOCKER_ERROR = False
 
 try:
-    from docker import Client
     from docker.errors import APIError, TLSParameterError
     from docker.tls import TLSConfig
     from docker.constants import DEFAULT_TIMEOUT_SECONDS, DEFAULT_DOCKER_API_VERSION
@@ -379,6 +378,19 @@ except ImportError as exc:
     HAS_DOCKER_ERROR = str(exc)
     HAS_DOCKER_PY = False
 
+# Client has recently been split into DockerClient and APIClient
+try:
+    from docker import Client
+except ImportError as exc:
+    try:
+        from docker import APIClient as Client
+    except ImportError as exc:
+        HAS_DOCKER_ERROR = str(exc)
+        HAS_DOCKER_PY = False
+
+        class Client:
+            pass
+
 DEFAULT_DOCKER_HOST = 'unix://var/run/docker.sock'
 DEFAULT_TLS = False
 DEFAULT_TLS_VERIFY = False
@@ -779,6 +791,10 @@ class DockerInventory(object):
         if config_path:
             try:
                 config_file = os.path.abspath(config_path)
+                # default config path is docker.yml in same directory as this script
+                # old behaviour is docker.yml in current directory. Handle both.
+                if not os.path.exists(config_file):
+                    config_file = os.path.abspath(os.path.basename(config_path))
             except:
                 config_file = None
 
@@ -813,7 +829,7 @@ class DockerInventory(object):
         # Parse command line arguments
 
         basename = os.path.splitext(os.path.basename(__file__))[0]
-        default_config = basename + '.yml'
+        default_config = os.path.join(os.path.dirname(__file__), basename + '.yml')
 
         parser = argparse.ArgumentParser(
             description='Return Ansible inventory for one or more Docker hosts.')