Allow LiveEvents to use env vars or an EC2 instance role
Allow LiveEvents to use env vars or an EC2 instance role

Fixes: CNVS-35832

This will allow us to configure the feature via consul w/o storing
credentials in the clear. This has been made possible by our new app
servers having an instance role we can use.

Test Plan:
- Enable Kinesis and Consul for docker
- Create the kinesis stream according to doc/live_events.md
- Add some dummy AWS credentials to the web and jobs containers
- Configure live events at `/plugins/live_events`
  - Check "Configure using Consul..."
  - Leave the rest blank
- Tail the kinesis stream using script/tail_kinesis:
  script/tail_kinesis http://kinesis.canvaslms.docker live-events us-east-1
- Create a course
- A live event should appear in the tailer window.

Change-Id: I136709697a7145491ec18fc712c6278f38f5eaef
Reviewed-on: https://gerrit.instructure.com/110200
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
QA-Review: Tucker McKnight <4b51b98c1d84aa97854d216bd061ce5d3eda8a28@instructure.com>
Product-Review: Tyler Pickett <3f20fcf21b1970c94966160adcf831eedd808a06@instructure.com>

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