* Add Nova interface-detach support

Add support to detach interface from a server through a DELETE on: /v2.1/{tenant_id}/servers/{server_id}/os-interface/{port_id}

The same operation with the OpenStack CLI is done with:
  nova interface-detach <server_id> <port_id>

* Update Nova's attachinterfaces extenstion docs.
Rename interfaceID to portID in the Delete request.

* Update Nova's attachinterfaces extenstion tests.
Rename interfaceID to portID in the Delete request test.