Make knife discoverer configurable

[abesto]

The goal is something like

(knife 
  (host "automatic.cloud_v2.public_hostname" "automatic.fqdn")
  (ip "automatic.cloud_v2.public_ipv4" "automatic.cloud_v2.public_ipv6" "automatic.ipaddress"))

which would

• Set Target.Host to the first set, non-empty value of automatic.cloud_v2.public_hostname, and automatic.fqdn
• Set Target.IP to the first set, non-empty value of automatic.cloud_v2.public_ipv4, "automatic.cloud_v2.public_ipv6, and automatic.ipaddress