List of plugins on /_nodes/plugins/ end point, like #2664

As #2664 got reverted and closed, here is a new pull request with an implementation, integrated in Nodes Info API.

Added parameter `plugins`.
Added end point `_nodes/plugins`.
Output information follows the struture below:
```json
{
  "plugins" : {
    "mandatory": [],
    "sites": ["head"],
    "nonsites": ["foo"]
  }
}
```