Add Latency and availability to DSSTATS

[bmfmancini]

As part of device polling

Latency and availability are available via the hosts table This data is useful in more detailed SLA reporting

 min_time: 0.01597
 max_time: 221.82298
 cur_time: 0.04792
avg_time: 0.16003

Also the current availability

availability: 100.00000

[TheWitness]

I think the way to address this would be to create a Graph Template that logs these things. @bmfmancini, can you do that?

[TheWitness]

So, two graph templates.