Add the outputs in to qiita_db.handlers.plugin.CommandHandler

[antgonza]

Currently we only return:

response = {
                'name': cmd.name,
                'description': cmd.description,
                'required_parameters': cmd.required_parameters,
                'optional_parameters': cmd.optional_parameters,
                'default_parameter_sets': {
                    p.name: p.values for p in cmd.default_parameter_sets}}

which is useless when we want to test that the names/outputs of a command are fine from the plugin itself.

From: qiita-spots/qp-qiime2#61