Simplify Node domain event specifications

This pull request simplifies the event specifications in the exposed Node domains API by removing a redundant single-property object in favor of just the value of that property. That is, it changes the exposed API from this: 
```
{
  "domain": "base",
  "events": [
  {
    "name": "log",
    "parameters": {
      "parameters": [
        {
          "name": "level",
          "type": "string"
        }
      }
    ]
  }
}
```
to this:
```
{
  "domain": "base",
  "events": [
  {
    "name": "log",
    "parameters": [
      {
        "name": "level",
        "type": "string"
      }
    ]
  }
}
```
This should be merged simultaneously with adobe/brackets#6154.

CC @jasonsanjose