Allow bq_dataset to use template

[ajimix]

Problem

Datasets are very useful to namespace different things (for example domains) in Google Bigquery. At the moment the dataset is a static string so if you want to use for example a dataset per host, you need to create multiple destinations manually.

Solution

Allow to use the same template variables as table_name_template but for bq_dataset.

So you can do something like this:

data_layout:
  table_name_template: '{{.event_type}}'
  google:
    bq_dataset: '{{.doc_host}}'