Add hosts endpoint.

[dongothing-dd]

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What does this PR do?

Add hosts endpoint. See #876.

Description of the Change

Added the hosts endpoint so I can get a host list without having to remember anything.

Alternate Designs

I considered adding to the "host" mode instead of creating a new "hosts" mode. Our endpoint is "hosts" so I used that naming convention.

Possible Drawbacks

N/A?

Verification Process

I tested in an Ubuntu VM with keys to the expert-services sandbox.

I ran flake8 tests via python -m tox -c tox.ini -e flake8
I ran and passed integration tests via python -m tox -c tox.ini -- -k hosts

Builds failed, but I assume that has to do with my environment rather than the change.

Additional Notes

I created a PR yesterday without the cassettes because it tested fine in my VM. The automated test failed here, so I added the cassettes today.

Release Notes

Added the hosts endpoints for the Datadog API. dog hosts list returns a list of all hosts. dog hosts totals returns counts of active hosts. Datadog documentation.

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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