[OpenStack/Subnet] Fix: Detach router interface before subnet removal (409 SubnetInUse)

[powerkimhub]

• Detach the subnet's router interface (GetRouter → DeleteInterface) before
  calling subnets.Delete, mirroring the teardown DeleteVPC already performs
• Add cleanupSubnetPorts to remove leftover non-VM ports (e.g. DHCP) still
  holding an IP allocation on the subnet, which also blocks deletion
• Fix kt driver's RemoveSubnet: it was checking a stale err instead of the
  actual delErr/waitErr from the subnet delete and wait calls, so failures
  were silently swallowed

Fixes #1823

[powerkimhub]

[after]

curl -sX DELETE -u admin:xxx http://localhost:1024/spider/vpc/powerkim-vpc01/subnet/subnet001 \
  -H 'Content-Type: application/json' \
  -d '{
    "ConnectionName": "openstack-config01"
  }'

{"Result":"true"}