Server/Plugins/Decisions: fix typo

This fixes this traceback:

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Bcfg2/Server/Core.py", line 818, in GetDecisions
    result.extend(plugin.GetDecisions(metadata, mode))
  File "/usr/lib/python2.7/dist-packages/Bcfg2/Server/Plugins/Decisions.py", line 34, in GetDecisions
    return getattr(self, mode).get_decision(metadata)
AttributeError: 'DecisionFile' object has no attribute 'get_decision'
```