Migrate away from swift namespace in attributes

Per this comment by core reviewer, https://review.openstack.org/#/c/67308/:

We should rename the attributes in this cookbook to match the attribute namespace pattern in use by the other cookbooks.

The current practice is to name the attributes in the following manner:

    default['openstack'][$short-cookbook-name]

As such, the proposal is to rename attributes into the following namespace:

    default['openstack']['object-storage']

As an example. the following attribute:

    default['swift']['state'] = {}

would now become:

    default['openstack']['object-storage']['state'] = {}

In addition, if the leaf attribute name contains "swift" in its name, it should be renamed to remove "swift" and moved to a more appropriate name.

For example:

    default['swift']['swift_secret_databag_name']

should become:

    default['openstack']['object-storage']["secret_databag_name"]