* 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
renamed plugins to contrib (they are not really plugins)
rewrote README.md to reflect new usage
added new dir to setup.py so it gets copied with installation, in views
of making using inventory scripts easier in teh future
