Add envoy_public_listener_port input var to mesh-task

[pglass]

Changes proposed in this PR:

This adds the envoy_public_listener_port to allow configuring the public listener port for Envoy. This defaults to 20000, which was the hardcoded value for the port.

• [x] Depends on #141 (test updates)
• [x] Depends on https://github.com/hashicorp/consul-ecs/pull/118

How I've tested this PR:

• Acceptance tests

How I expect reviewers to test this PR:

Checklist:

• [x] Tests added
• [x] CHANGELOG entry added