Add options for iframe height and width

[nobane]

Adds optional height/width for iframes. The old format is still supported but to specify a height or width, you can do:

{
  "checks": {
    "example-check": {
      ...
      "graph": "iframe:{ 'src': 'http://example.com', 'height': '400px' }",
      ...
    }
  }
}

Closes sensu/uchiwa#775