Support for multiple Graphites

[n0740]

Adds ability to check more than one Graphite.

Describe them in config:

{
    ...
    // Drop params for single graphite:
    // "graphite_url": "",
    // "graphite_username": null,
    // "graphite_password": null,

    // Then set the list:
    "graphites": [
      {
         "name": "default",
         "url": "http://localhost",
         "username": "username",
         "password": "password"
      },
      {
        "name": "another-one",
        "url": "http://another-one",
      }
    ],
    ...
}

And set alets for each Graphite by it's name:

{
  ...
  "alerts": [
    {
      // If param "graphite" is skipped alert will check the "default"
      // "graphite": "default".
      "name": "Memory",
      "query": "aliasByNode(collectd.*.memory.memory-free, 1)",
      "interval": "10minute",
      "format": "bytes",
      "rules": ["warning: < 300MB", "critical: > 200MB"]
    },
    {
      // Alert for another graphite
      "graphite": "another-one",
      "name": "Memory",
      "query": "aliasByNode(collectd.*.memory.memory-free, 1)",
      "interval": "10minute",
      "format": "bytes",
      "rules": ["warning: < 300MB", "critical: > 200MB"]
    },
  ]
  ...
}