Add config validation utility to Fargate Agent

[joshmeek]

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• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

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What does this PR change?

This PR adds a function to the fargate agent for validating the configuration that it was provided. For example something like this could happen:

agent = FargateAgent(
    aws_access_key_id=os.getenv("AWS_ACCESS_KEY_ID"),
    aws_secret_access_key=os.getenv("AWS_SECRET_ACCESS_KEY"),
    region_name="us-east-1",
    cpu="1024",
    memory="8192",
    executionRoleArn="my_arn...",
    containerDefinitions=[
        {
            "logConfiguration": {
                "logDriver": "awslogs",
                "options": {
                    "awslogs-region": "us-east-1",
                    "awslogs-group": "my_group",
                    "awslogs-stream-prefix": "prefect",
                },
            },
        }
    ],
    networkConfiguration={
        "awsvpcConfiguration": {
            "assignPublicIp": "ENABLED",
            "subnets": ["subnet-0773697d1a87dd105"],
            "securityGroups": [],
        }
    },
)

agent.validate_configuration()

And this validate_configuration function will test to see if the agent has the proper setup to register a task definition and run the task.

[2020-06-11 20:01:12,321] INFO - agent | Testing task definition registration...
[2020-06-11 20:01:12,499] INFO - agent | Task definition registration successful.
[2020-06-11 20:01:12,499] INFO - agent | Testing task run...
[2020-06-11 20:01:13,409] INFO - agent | Task run ...my_task_run... successful.

Why is this PR important?

It is not uncommon for questions around the fargate agent to be based around configuration and usually these are only discovered after getting it up and running. This can hopefully alleviate some issues before attempting to run a flow by validating ahead of time.

I'm also now thinking that maybe every agent should have its own validate_configuration to verify that it is configured properly.

[codecov]

Codecov Report

Merging #2768 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.