Give warning of incipient overload in the mlab-ns output

[pboothe]

We would like to provide a little nuance in our JSON output, by adding an extra field which indicates whether the system is currently near overload. Tests can still be performed when the system is near overload, but an interactive client will likely not want to run a test due to the increased probability of providing a bad user experience.

We will need to plumb this new status of through the monitoring and mlab-ns systems, which will likely be a lot of work. The very first step in this work, however, is hopefully easy: add the extra key/value pair "status": "healthy" to the JSON output. Then, development of better clients can begin ASAP, because the API will be already there, and our work on building up all the backend plumbing can proceed asynchronously in parallel.

The sequence of feature changes is:

• 1. Add the extra key/value pair "status": "healthy" to the JSON output
• 2. Add a new status to the monitoring system of "near-overload" to track not just whether the system is up, but also whether testing is a good idea right now.
• 3. Add this new status to the data model for each sliver.
• 4. Parse the new monitoring data into datastore and memcache
• 5. Change the JSON output to use the newly-dynamic value for "status" instead of being hardcoded to "healthy"