Add health check information to inventory

[karencfv]

As a first step to implement RFD 612, we'll be needing to include the following information to the sled agent inventory:

• Missing sleds
• SMF services in maintenance
• Unhealthy zpools
• Sagas running for more than an hour (the time here is arbitrary)

[davepacheco]

I imagine that "missing sleds" is not adding anything new, just noticing that a sled agent or SP is missing from the inventory collection.

[karencfv]

True! I'll remove that one