Fix attach_interfaces tests of Nova v3

Tempest's interfaces_client calling Nova "os-interface" v2 API was fixed
in I3db0381d48919c62f3f3c6d60fc14cf4dee9c4a3 .
However, there is still a same bug about v3 API. This commit fixes it.

NOTE: API and POST parameter was changed in Nova v3.
 v2: {API: os-interface, parameter: interfaceAttachment}
 v3: {API: os-attach-interfaces, parameter: interface_attachment}

Change-Id: I7b5b9419366d779bf07dd19383f28550b9d6f369
Closes-Bug: #1271642
Merge "Fix attach_interfaces tests of Nova v3"