adding choices of prop as an option

since prop was just a placeholder .. i was thinking of replacing it with options. with this change it looks like this(i can add helptext with each option once i know what to add )


(.slenv)~/office/softlayer-python (issue437) > sl metadata --help
Usage: sl metadata [OPTIONS]

  Find details about this machine.

Options:
  --backend_ip
  --backend_mac
  --datacenter
  --datacenter_id
  --fqdn
  --frontend_mac
  --id
  --ip
  --network
  --provision_state
  --tags
  --user_data
  --help             Show this message and exit.

(.slenv)~/office/softlayer-python (issue437) > sl metadata --tags
Cannot connect to the backend service address. Make sure this command is being ran from a device on the backend network.
(.slenv)~/office/softlayer-python (issue437) > sl metadata --ip
Cannot connect to the backend service address. Make sure this command is being ran from a device on the backend network.



This line needs a couple spaces to pass the style tests and to avoid smashing the additional note to the end of the normal help message. For example:
```python
        attrhelp + " (multiple occurrence permitted)"
                  ^ ^
```