Add tags and mode to network db model. Replace existing type with
flavor. Introduce 4 different flavors. Each flavor has mode, link,
mac_prefix, tags. Supported flavors are CUSTOM, IP_LESS_ROUTED,
MAC_FILTERED, PHYSICAL_VLAN and are hardcoded in db models.

Introduce new function that returns flavor's mode, link, mac_prefix,
tags.

In settings only DEFAULT_ROUTING_TABLE, DEFAULT_BRIDGE and
DEFAULT_MAC_PREFIX, DEFAULT_PRIVATE_BRIDGE exist.

Introduce FIELD_POOL_MAP to map flavor fields to pool tables.

Make snf-manage network-create command as flexible as possible. It is
now able to create a network based on an existing flavor and overide
defaults by passing mode, link, mac_prefix, tags option. Resource pools
cannot be used by CUSTOM flavors. Currently MAC_FILTERED and
PHYSICAL_VLAN use MacPrefixPoolTable for mac_prefix and BridgePoolTable
for link accordingly and cannot be overriden.

API blocks creation of public networks. Introduce new setting
API_ENABLED_NETWORK_FLAVORS to indicate which flavors can the end-user
use to create private networks.

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