Add identifiers field to Bolt task metadata schema

[hsnodgrass]

This commit adds the identifiers field to Bolt task metadata. A brief description of why an identifiers field would be useful is included in the tasks/README.md update. A more concrete example is writing Bolt tasks to help with security compliance. In order to map tasks to compliance rules, you either need to name your task after a rule or use comments in the implementation script. With the new identifiers field, you could add a keys under identifiers showing details about the security controls implemented.

"identifiers": {
  "compliance_controls": {
    "cis": ["some_control", "another_one"],
    "stig": ["V-000000"]
  }
}

This PR addresses issue #159

[donoghuc]

Rolling out something like this needs to happen in the task spec and have a path forward for all task runners (most notably PE).

[hsnodgrass]

Rolling out something like this needs to happen in the task spec and have a path forward for all task runners (most notably PE).

Is this something I can help with? If so, where should I look?

[hsnodgrass]

@donoghuc I've updated this PR based on the discussion in #159

[hsnodgrass]

@donoghuc anything else need to be done on this before it gets merged?

[donoghuc]

nope, looks good to me. I think we've waited long enough to allow for other opinions.