#538 - variables do not conform to Puppet 3.4 naming rules

I simply replaced all variable names beginning with an underscore using:
```
find . -name \*.pp | xargs sed -i -e 's/$_/$my_/g' -e 's/${_/${my_/g'
find . -name \*.erb | xargs sed -i -e 's/@_/@my_/g'
```

Not exhaustively tested, but works for my vhost config with puppet 3.3.2 and 3.4.0rc2.