IP pool related objects, opcodes and constants

Config objects:
 * Introduce L{Network} with
  - IPv4 network field (mandatory)
  - IPv4 gateway, IPv6 (network/gateway), mac prefix, type (optional)
 * Modify existing config objects to support networks:
  - Add new slot 'network' to L{NIC} config object
  - Add new slot 'networks' to L{NodeGroup} config object

Opcodes:
 * Introduce new opcodes for networks
  - add/remove/modify/query/connect/disconnect.
 * In InstanceCreate/InstanceSetParams add conflicts_check option

Constants:
 * INIC_PARAM 'INIC_NETWORK'
 * NIC_IP_POOL for automaticaly obtain an IP from a pool
 * NETWORK_TYPE_PUBLIC/PRIVATE for network types

Checking of network_type handled by the opcode parameter validation.
Introduce _CheckCIDR*Notation() functions for network parameters validation.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>