Added variable to panel-variation for body bg color with defaults to transparent

The idea behind this is to let someone use more panel to extend existing ones like : 

```
@panel-clear-border : #ccc; 
@panel-clear-text : #333; 
@panel-clear-heading-bg : #fff; 
@panel-clear-body-bg: #fafafa;

.panel-clear {
  .panel-variant(@panel-clear-border; 
          @panel-clear-text; 
          @panel-clear-heading-bg; 
          @panel-clear-border; 
          @panel-clear-body-bg);
}
```