fix: docker health check status not updating

[thielepaul]

I wanted to use the new health check status added in #3640 and noticed two issues that are fixed in this PR.

• The state of the health status is never updated, instead it is read only once (at container start or at cadvisor start)
• Containers without health check and unhealthy containers both have the value 0 in the metric (I changed it to -1)

[thielepaul]

@dims can you take a look, if these changes make sense to you?

[dims]

@thielepaul do you mind adding tests?

[thielepaul]

@thielepaul do you mind adding tests?

I added a test for the value conversion in prometheus.go, however, I am unsure how to write a good test for the change in handler.go. If you think another test makes sense, please give me some pointers on where to add it.

[dims]

here? https://github.com/google/cadvisor/tree/master/integration/tests/api

[thielepaul]

here? https://github.com/google/cadvisor/tree/master/integration/tests/api

Thank you! I added an integration test.
Do you have any other comments?

[thielepaul]

@dims can you please trigger the tests again? I added a tag to the gcr.io/k8s-staging-test-infra/bootstrap image, so the same version that was previously is used and the tests hopefully pass.