* 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
f9a179f770