Update cluster-setup docs

[guillermoap]

Hello, I've been using frontera for the last couple of months and have found that in some places the docs are not up to date. In this case the setup-cluster docs.

If I try to run the dbworker as specified in the setup cluster doc on line 130, by running:
python -m frontera.worker.db --config src.config.db_worker --no-scoring --no-incoming --partitions 0,1

I get the following output:

dbworker_batch_1        |              [--partitions [PARTITIONS [PARTITIONS ...]]] --config CONFIG
dbworker_batch_1        |              [--log-level LOG_LEVEL] [--port PORT]
dbworker_batch_1        | db.py: error: argument --partitions: invalid int value: '0,1'

By trial and error I found out that the current correct way to initialize the dbworker with a specific number of partitions is by running the following:
python -m frontera.worker.db --config src.config.db_worker --no-scoring --no-incoming --partitions 0 1

As well the CRAWLING_STRATEGY config var that is specified in the doc, on line 91, if you config that var the specified crawling strategy is not taken into account by frontera. So I looked into the default_settings file, on line 77, to see how to correctly set that var and there the var that does that is named STRATEGY. When I made that change the strategy started working as expected.

So to sum everything up, I've just updated the docs to reflect this changes.

[jpbalarini]

👍

[sibiryakov]

thank you very much!