shellvar uses variable as namevar

this makes it impossible to set the same variable in different files/modules. Here's a minimal example:
```puppet
shellvar { 'x-port':
  variable => 'port',
  value => '2034',
  target => '/tmp/x'
}
shellvar { 'y-port':
  variable => 'port',
  value => '2034',
  target => '/tmp/y'
}
```