use node.set for future chef 11 compatibility

small update for chef 11, fixes this type of warning:
```
2013-01-30T22:05:17-05:00] WARN: Setting attributes without specifying a precedence is deprecated and will be
removed in Chef 11.0. To set attributes at normal precedence, change code like:
`node["key"] = "value"` # Not this
to:
`node.set["key"] = "value"` # This
```