Buildkite Pipeline - AWS infra related improvements

[thundergolfer]

My Buildkite setup is now at a stage where I can run bazel test //... and have things pass 😄, but things are far from perfect.

Improvements to be made:

1. Currently cannot access like buildkite-agent instances. The Buildkite CloudFormation template has something about ssh keyPair that can supposedly address this
2. EC2 Instance setup currently happens in the bootstrap_agent.sh script. I think it would be much better to bake this setup into an AMI and then use that as the agent AMI instead of the base buildkite-stack AMIs. Having 1 from this list will help achieve this.
3. Lower Priority: I had to go up from t2-nano to t2-small to avoid a JVM out-of-memory error. See if I can do some tuning or find a better instance type for these BK agents.

[thundergolfer]

4. My buildkite-agent instances are terminating after every completed step!!! Need them to stay up for at least 5 minutes or so waiting to see if it can pick up another step. Jeezus.