Couple of updates to cloud.boot_instances()

* db.get_node_objects() was responsible for creating node objects.
  Its implementation was very EC2 specific. Hence, now
  boot_instances() will associate IPs and create node objects

* cloud.boot_instances() now associates IPs to create updated node
  objects.

get_node_objects() has been removed.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>