Also calculate external subnets when check cidr whether in use
Also calculate external subnets when check cidr whether in use

The external network be shared by all tenant. When it associated
to a tenant router the cidr of it's subnets cannot overlap with
tenant subnets', otherwise it will lead to bug #1903433. The related
fix patch: https://review.opendev.org/761829

So, in tempest, the external subnets need was considered when check
cidr whether in use.

Related-Bug: #1903433
Change-Id: Ief84251adb0368533570d9e9f6630de92e27c5c3

openstack/tempest,openstack/tempest