Add quality engine health check

[gothub]

Add a check for the quality engine to the monitor facility used by the NCEAS compute team.

The check should determine that requested quality reports are being fetched and that quality reports are being created for the most recent content.

This health check is a bit different than an audit (described in #176), as this doesn't verify content or initiate any action, it just checks that the system is up and responding.

[gothub]

The 'k8s best practices' way to implement checks is with readiness and liveness probes as outlined herehttps://cloud.google.com/blog/products/gcp/kubernetes-best-practices-setting-up-health-checks-with-readiness-and-liveness-probes.

This may be overkill, but this method can also be plugged into the container startup so that k8s knows exactly when to make a pod available.

One way to use these probes is to have a metadig-engine REST endpoint (monitor?) that tells the controller to send out all necessary probes, wait for a certain time and then report back with the results.

Probes/checks could be enabled for:

• the controller itself to request an existing quality report
  • this may also check that metadig-workers are available
• metadig-scheduler to ensure that new reports can be generated
• metadig-scorer to ensure assessment graphs can be generated