Add API handlers for DHCP subnets and their hosts

This patch adds the API handlers for DHCP subnets and DHCP subnet
hosts.

This patch adds comprehensive unit tests for the DHCP subnets.

Change-Id: If93d2164e48f610a9c6d5e1109cd9c75a51b14ce
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>