Enhancement: Add sysinfo.json option to return number of mesh nodes

[bwarden]

For automated monitoring of a node's health and its connected mesh's health too, it would be nice to get a snapshot of how many mesh nodes exist via sysinfo.json. This snippet would do the job:

-- MESH NODES COUNT
if string.find(nixio.getenv("QUERY_STRING"):lower(),"nodes=1") then
        info['nodes']=aredn_info.getOLSRInfo()
end

and adds to the returned object:

  "nodes": {
    "nodes": "335",
    "entries": "1186"
  },