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'
}
```
and its error:
```shell
igalic@levix /tmp % puppet apply port.pp
Notice: Compiled catalog for levix.box in environment production in 0.08 seconds
Error: Cannot alias Shellvar[y-port] to ["port"] at /tmp/port.pp:10; resource ["Shellvar", "port"] already declared at /tmp/port.pp:5
```