Do not query all DF ports when deleting a network
Do not query all DF ports when deleting a network

Currently, when deleting a network, all ports are queried and each
is checked, whether it is a DHCP port for a subnet of this network.
Instead, iterate through the subnets of this network, and get the
DHCP port for each.

Change-Id: Ied411685e8e6e83cba0bc3df0b8b4f06fa991e43
Closes-Bug: 1544464

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow