display prop choices in help with examples

~/office/softlayer-python (issue437again) > sl metadata --help
Usage: sl metadata [OPTIONS] PROP

  Find details about this machine

  PROP Choices
  *backend_ip
  *backend_mac
  *datacenter
  *datacenter_id
  *fqdn
  *frontend_mac
  *id
  *ip
  *network
  *provision_state
  *tags
  *user_data
  
  Examples :
  sl metadata backend_ip
  sl metadata backend_mac
  sl metadata datacenter
  sl metadata datacenter_id
  sl metadata fqdn
  sl metadata frontend_mac
  sl metadata id
  sl metadata ip
  sl metadata network
  sl metadata provision_state
  sl metadata tags
  sl metadata user_data

Options:
  --help  Show this message and exit.

  These commands only work on devices on the backend SoftLayer network. This
  allows for self-discovery for newly provisioned resources.

To avoid specifying the options twice, use META_CHOICES here.