add Diplomat::Role for Consul ACL roles in 1.5.x

[scalp42]

Consul 1.5.0 introduced a number of changes (https://github.com/hashicorp/consul/blob/master/CHANGELOG.md#150-may-08-2019):

• ability to specify AccessorID and SecretID (need valid UUIDs though)
• ability to bind policies to roles (Diplomat::Role is added here)

This PR fixes the issue #195.

I'll open an issue for ACL Auth Method HTTP API and ACL Binding Rule HTTP API support but it won't be covered in this PR as I don't have access to a k8s cluster right now.

Please do not merge this PR until WIP is removed from subject as testing is in progress.

Thanks for reviewing it @pierresouchay 🇫🇷

[scalp42]

I've been test driving this with Chef, looking good but we're not using roles (due to https://github.com/hashicorp/consul/issues/5221#issuecomment-491162829) so YMMV

[scalp42]

Feel free to review @pierresouchay

[scalp42]

Thanks for the review @pierresouchay. PR is updated with suggested changes.

[pierresouchay]

@scalp42 rubocop has been upgraded, hence the not-working linting. I fixed it: https://github.com/WeAreFarmGeek/diplomat/commit/3f9465c0ba20e7ad60c788f78fa13dd24970636e

Please re-push force, it should work next time

[scalp42]

Done.