[ML] Allow index patterns containing commas in recogniser modules

[jgowdyelastic]

When supplying an indexPatternName override to the modules/setup endpoint, it can now contain a comma separated list of indices. These will be split and added as an array to the datafeed's indices.

Example usage:

POST <kibana server>/api/ml/modules/setup/siem_auditbeat_ecs
{
  "indexPatternName": "index1, index2"
}

Also fixes an issue where only indexes was checked for when reading the datafeed json configs.
All modules currently in ML use indexes, probably due to copying and pasting modules when creating new ones.
Now either indices or indexes can be used in a module's datafeed configs.

Checklist

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[elasticmachine]

Pinging @elastic/ml-ui

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[walterra]

LGTM

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[alvarezmelissa87]

LGTM ⚡️