Check router interface not in use properly
Check router interface not in use properly

_confirm_router_interface_not_in_use uses subnet cidr
and does not account for other subnets with same cidr.
Need first check that router actually has interface from this
subnet.
Also _confirm_router_interface_not_in_use is more expensive
than check for router ports, so its better to perform it after
confirming router has interface from this subnet.

Closes-Bug: #1906508
Change-Id: I406703209cdd720f6fea1a1f15711dc527e1cdb0

mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron