Fix default docker-storage-driver

The API doesn't set a default value for docker-storage-driver
only the client passes a default.

We should remove the default from the client
magnumclient/osc/v1/cluster_templates.py:152: default='devicemapper',
magnumclient/v1/baymodels_shell.py:87: default='devicemapper',
magnumclient/v1/baymodels_shell.py:89: 'overlay. Default: devicemapper'))
magnumclient/v1/cluster_templates_shell.py:138: default='devicemapper',
magnumclient/v1/cluster_templates_shell.py:140: 'overlay. Default: devicemapper'))

AND

update the api-ref correctly.
source/parameters.yaml: ``overlay``. The default is ``devicemapper``.

AND

update docs too:
source/user/index.rst: and 'overlay'. The default is 'devicemapper'.

Document that for fedora-{swarm, swarm-mode, kubernetes} the default is devicemapper but in the driver implementation not in the API.