network, subnet cleanup

* network response with admin_state_up as None
* subnet response with enable_dhcp as None
* bugfix: load allow_overlapping_ips from quantum config
* bugfix: on create_subnet, specify id as named argument for network_find
* elaborate create_subnet test
Bugfix for subnet deletion

Avoid foreign key failure from sqlalchemy by cascading the subnet
deletion.
Merge pull request #83 from asadoughi/subnet_delete_fix

Bugfix for subnet deletion
Merge pull request #82 from asadoughi/net_subnet_cleanup

network, subnet cleanup