Update digital_ocean.py to provide "_meta" header

Dynamic inventory plugins are supposed to provide a "_meta" header to speed things up. This plugin doesn't have that header. I've added the header for the --list option. The speed increase is pretty big even when this plugin is used alone.

If you have multiple dynamic inventory plugins, and one of them doesn't do "_meta", things get EXTREMELY slow, especially when you have lots of hosts. See issue #9774

https://github.com/ansible/ansible/issues/9774