[ECS/EC2] [request]: Support joining ECS cluster without ECS_CLUSTER env var

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**Tell us about your request**

Support a way for the ECS agent to read config on startup via ec2 instance tags, metadata, or from s3

**Which service(s) is this request for?**

ECS on EC2

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

Currently every ECS container instance requires specification of custom userdata in order to join them to the correct ECS cluster. ECS could support a way for the instance to join a cluster without the ECS_CLUSTER env var being specified in userdata. For example, there could be a "reserved" ec2 instance tag that ECS always looks for if it doesn't find ECS_CLUSTER.

**Are you currently working around this issue?**

specifying ECS_CLUSTER in userdata

The request identifies ECS on EC2 and the ECS agent, but names no repository files, tests, or entry points. Begin by locating the agent's startup configuration handling and compare the proposed instance-tag, metadata, and S3 sources; done means an ECS container instance can join the intended cluster without ECS_CLUSTER in userdata.

Written by the indexing model from the issue text.