Add path environment for execution

When using this module with puppet on the vagrant box hashicorp/precise64, I experienced a bug with executing my puppet config:
```
...
ssh::resource::known_hosts { 'vagrant':
  hosts => 'host.xyz.org',
  user => 'vagrant',
}
...
```

Caused the following error:

Parameter unless failed: '[ -f /home/vagrant/.ssh/host_hash ] && [ `sha512sum /home/vagrant/.ssh/host_hash | awk '{ print $1 }'` = `echo 'host.xyz.org' | sha512sum | awk '{ print $1 }' | sha512sum | awk '{ print $1 }'` ]' is not qualified and no path was specified. Please qualify the command or specify a path.