Pre-created ports get deleted on interface-detach

[Tomasz Głuch]

Neutron ports which exist prior to being attached to instance are removed on interface-detach.

Way to repeat:
1) Create port (neutron port-create <NETID>)
2) Create instance (nova boot ...)
3) Attach port created in 1) to instance (nova interface-attach --port-id <PORTID> <INSTANCEID>)
4) Detach port (nova interface-detach <INSTANCEID> <PORTID>)

Expected behaviour:
Port should still exist, as it had already existed prior to attaching.

Observed behaviour:
Port has been removed (neutron port-show <PORTID> :
Unable to find port with name 'PORTID'

This issue is similar to https://bugs.launchpad.net/nova/+bug/1158684
Tested on IceHouse.

[Davanum Srinivas (DIMS) (dims-v)]

Review https://review.openstack.org/77043 for bug #1158684 will fix this scenario as well.