Allow dataset to not load a template

There are some datasets which allow to select and configure and input directly on the Agent side and with it, configure the data_stream.dataset field. Because of this, loading a template does not make sense in this context.

Our first example of this is the `log` package. It allows to tail any log file and the user can configure where the log should be shipped to. It currently loads a template into `logs-log.log-*`, but the template that actually applies to the data is the `logs-*-*` template except by chance `log.log` was set as dataset name.

We currently validate that dataset MUST contain the `data_stream.*` fields. But in this case the creator of the package should be able to set a flag in the `daaset/manifest.yml` file like `template: false` (better name suggestions welcome) which disables this validation. And then on the Kibana side, if no fields.yml exists, no template is created.