Add a termination_reason flag to distinguish between manual and scheduled termination

[maurodoglio]

At the moment there's no way to know if a cluster was manually terminated or it reached its scheduled end of life.

[jezdez]

@maurodoglio I'm tackling this as part of #201 interestingly, did you read my mind?

[jezdez]

I'm actually not doing that as part of #201 anymore. Sorry for the noise.

[maurodoglio]

If I had to couple this work with another issue, that would be https://github.com/mozilla/telemetry-analysis-service/issues/264 😄

[jezdez]

@maurodoglio 👍