Use Login API instead of the CLI

[pglass]

Changes proposed in this PR:

These changes match https://github.com/hashicorp/consul-ecs/pull/115 to use the Login API. This only required removing the ExtraLoginFields option from the ConsulLogin config object. I also updated the consul_ecs_config var to support passing the additional login fields.

How I've tested this PR:

• Acceptance tests
• Ran the examples/mesh-gateways example

How I expect reviewers to test this PR:

👀

Checklist:

• [x] Tests added
• [x] CHANGELOG entry added
• [x] Depends on https://github.com/hashicorp/consul-ecs/pull/115

[cthain]

Oh, just realized that this probably needs a changelog entry since it adds the consulLogin field and associated validation to the config file.

[pglass]

I updated gateway-task in recent commits.