Allow custom openstack service type

For our setup to work, we need the `openstack_service_type` to be customizable. This PR makes it possible to define it in your bosh manifest. The `Swift::Node` will accept this option as it passes all identity options straight through to `Swift::Identity` ([link](https://github.com/anynines/cf_swift_service/blob/master/lib/swift_service/swift_node.rb#L48)) and then to `Fog::Identity` ([link](https://github.com/anynines/cf_swift_service/blob/master/lib/swift_service/identity.rb#L15)).

It then ends up in `Fog::Identity::Openstack` where we can see that the `openstack_service_type` option is recognized and by default set to `['identity']` ([link](https://github.com/fog/fog/blob/master/lib/fog/openstack/identity.rb#L189))

Therefore I have added `['identity']` to the example manifest as the default value for `openstack_service_type`. Things will still work if the option is left out of the manifest, making this fix backwards compatible.