Avoid starting the agent if the api key is too short

[masci]

Note: Please remember to review the Datadog Contribution Guidelines
if you have not yet done so.

What does this PR do?

Performs a sanity check before starting the agent to avoid collecting anything if the api key is obviously wrong.

Enhance the message on the status page when an api key is invalid

Motivation

The agent might start collecting metrics but never flush if users paste a wrong api key

[xvello]

Fixes DataDog/docker-dd-agent#250

[xvello]

LGTM !

[ISauve]

Note: the process-agent and trace-agent treat a missing/empty/shortened API key differently than the dd-agent does, and since supervisor does not have the concept of “dependencies” each process is on its own. We observed that an empty API key lead the process-agent to start as usual, an API key that was too short made both processes start, and a missing API key (line completely commented on in conf.d) meant neither started. This PR doesn't affect those agents at all, so the observed behaviour is not a bug that introduced here - it's just how the other agents work.