Handle boolean caps sent as strings

When users send in a boolean desired capability as a string we want to make it an actual boolean, so server logic can act on it.

With this update, capabilities such as

```javascript
var caps = {
  app: appPath,
  autoLaunch: "false",
  // ...
};
```

will function just like

```javascript
var caps = {
  app: appPath,
  autoLaunch: false,
  // ...
};
```

Resolves #3155