Allow variables in YAML definition of sql batches

[mkrcah]

Sometimes it is needed to perform several sql batches with queries with are almost identical but differ in one parameter. For example:

- query: SELECT * FROM foo WHERE bar = 'Spark rocks!'
  output: spark.csv
- query: SELECT * FROM foo WHERE bar = 'Impala rocks!'
  output: impala.csv
- query: SELECT * FROM foo WHERE bar = 'Hadoop rocks!'
  output: hadoop.csv

Especially for complex sql queries, it would be very useful to provide a mechanism in the YAML batch-sql config to define a for-loop and provide a placeholder in the query and output parameter.

For example, we might follow the path that Ansible chose for loops:

- query: SELECT * FROM foo WHERE bar = '{{item}} rocks!'
  with_items:
  - "Spark"
  - "Impala"
  - "Hadoop"
  output: {{item}}.csv

[DonDebonair]

This is already possible, but it wasn't in the README yet ;) In fact, I was just writing a paragraph about it.

[mkrcah]

Haha, I thought you are doing some heavy refactoring :)

Hopefully, this hard replacement of {{ item }} would suffice, let's see in practice.

[DonDebonair]

There was some heavy refactoring, but the old construct you created for repeatable queries is still there. I think we should roll with it, and make it more comprehensive only when the need arises.

[mkrcah]

Agreed