LUCreateInstance: Retrieve IPs from an IP pool

A new ip keyword is added ("pool") to signify that LUCreateInstance should get
the instance's IP from an IP pool (rather than manually or by DNS resolution).

IP and link checks are re-ordered so that a NIC's link is available at the time
of IP address validation.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>