In batch template, access to the defined dbrp is missing

A batch template can be bound to a dbrp in different ways:
- explicitly via the `dbrp` keyword inside the template
- via the command line (`kapacitor define cpu_alert -template generic_mean_alert -vars cpu_vars.json -dbrp telegraf.autogen`)
- via the `dbrps` field of a task definition

The problem is that the same dbrp must also appear in the `query` definition, after the `FROM`.

For a batch template to be used with different dbrps, the defined dbrp must be available somehow from the template, for instance as a `dbrp` variable:
```
batch
|query('select ... from ' + dbrp + ' where ...)
.every(1m)
```

Start by tracing how batch templates receive dbrp through the template keyword, the kapacitor define command's -dbrp option, and task definitions' dbrps field, then how the query's FROM clause is represented. Done requires an agreed mechanism for a template to access its selected dbrp so the same template can construct queries for different dbrps.

Written by the indexing model from the issue text.