agent start CLI command now allows for kwargs

[joshmeek]

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Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (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?

Closes #1734
prefect agent start now accepts kwargs. This is useful for users when providing the configuration options for the Fargate Agent.

e.g.

$ prefect agent start fargate cpu=$CPU memory=$MEMORY networkConfiguration=$NETWORK_CONFIGURATION

Why is this PR important?

Having to provide kwargs in the boto3 camel casing convention could potentially lead to conflicts in environment variable names when using external tooling (e.g. Terraform) to deploy the Agent.

Instead of having to do:

$ export memory=512
$ prefect agent start fargate

There is now the option to use custom names:

$ export AGENT_MEMORY=512
$ prefect agent start fargate memory=$AGENT_MEMORY

[joshmeek]

Interesting, looks like a few tests are failing on 3.5 for unrelated reasons:

FAILED tests/tasks/google/test_bigquery.py
FAILED tests/tasks/google/test_gcs_copy.py
FAILED tests/tasks/google/test_gcs_upload_download.py

[cicdw]

Looks like google made some python releases very recently: https://github.com/googleapis/google-cloud-python/releases/tag/storage-1.23.0

You can either:

• investigate the changes and whether, e.g., support for 3.5 has been dropped
• bound our google packages above in requirements to temporarily avoid the error

[joshmeek]

Going to change the bounds now. Doesn't appear they dropped support for 3.5 but this issue googleapis/google-cloud-python#9786 is relevant. Not sure why it's only affecting the tests for 3.5 though.

[codecov]

Codecov Report

Merging #1737 into master will increase coverage by 0.01%.
The diff coverage is 100%.

[cicdw]

awesome, i think this is a significant improvement