add a hashivault_token_role_list module

[Agalin]

This module lists roles configured in a specified token auth engine.

This module supports check mode (even though it does not perform any write operations) to ensure a whole playbook can run in check mode and proper data is still returned from this module for further tasks to use.

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[Agalin]

hvac raises an InvalidPath exception when no roles exist (but mount point is correct) with empty errors and None custom message. I believe it may be a bug in their implementation but added a workaround for that.

[TerryHowe]

Thanks!