Update IP pool management design doc

Update IP pool managenent design doc to be consistent
with the implementation.

 * Add new NIC parameter: 'network'
   Can be None for backwards compatibility.
 * Introduce the term 'netparams'
   The NIC inherits netparams (mode, link) as its nicparams
   if assinged to a network. Netparams are defined during
   network connection to a nodegroup.
 * Introduce the term 'Conflicting IPs'
   Ensure IPs uniqueness inside nodegroups.
 * Update 'Hooks' section.
 * Update 'Hook variables' section
 * Update 'Userland interface' to reflect the implementation

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>