interface-attach: does not prevent attaching multiple ports to same network on same instance

[Aaron Rosen]

When booting a vm you are only allocated to have one port on each network attached to the instance. Currently, you can add multiple interfaces on the same network via interface-attach. This is a regression from grizzly where this was working correctly previously.

[Aaron Rosen (arosen)]

arosen@arosen-desktop:/opt/stack/nova$ nova list
/usr/lib/python2.7/dist-packages/gobject/constants.py:24: Warning: g_boxed_type_register_static: assertion `g_type_from_name (name) == 0' failed
  import gobject._gobject
+--------------------------------------+------+--------+------------+-------------+----------------------------+
| ID | Name | Status | Task State | Power State | Networks |
+--------------------------------------+------+--------+------------+-------------+----------------------------+
| 97e1dbee-b940-428e-a912-57fba784fa5f | vm1 | ACTIVE | None | Running | private=10.0.0.3, 10.0.0.4 |
+--------------------------------------+------+--------+------------+-------------+----------------------------+