Add the logic to handle the another level of nested resource

This patch enables midonet_extionsion to handle the nested resources
below the one level from the parent, or the target resource. The parent
resource should have children's names and attribute maps in its
CHILD_RESOURCE_ATTR_MAP.

Change-Id: Iec4d7f99c16bd80f999f64b08324db2d78f346a5
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Modify Tunnelzone extension to use CHILD_RESOURCE_ATTRIBUTE_MAP

This patch modifies Tunnelzon to use CHILD_RESOURCE_ATTRIBUTE_MAP. Now
it doesn't need to implement its own get_resources.

Change-Id: Id90a568cce069ad56045d573b1984247d7967f3a
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>