Fix DefaultSubnetPool API test
Fix DefaultSubnetPool API test

As default subnetpool is a unique resource in the cloud, it needs to be
cleaned after each test is done. This patch adds a cleanup call to
DefaultSubnetPool tests in order to delete created default subnet pool.

Change-Id: I4c963d0d0e9910f7047061b51feb36c8a19de65c
Closes-bug: #1709938

huntxu/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,noironetworks/neutron,eayunstack/neutron,huntxu/neutron,openstack/neutron,openstack/neutron,eayunstack/neutron,noironetworks/neutron