use different variable for namevar

this makes it possible to actually set the same variable in two
different files:
```puppet
   shellvar { 'x-port':
     variable => 'port',
     target   => '/etc/x',
     value    => '2034',
   }
   shellvar { 'y-port':
     variable => 'port',
     target   => '/etc/y',
     value    => '8107',
   }
```
This pull request fixes #120