router-interface-delete command has incorrect help

[George Shuklin]

router-interface-delete says it need router and INTERFACE, while command itself requires subnet.

usage: neutron router-interface-delete [-h] [--request-format {json}]
                                       ROUTER INTERFACE

neutron router-interface-delete a88edee2-f4e0-47d1-977a-f0714c305fd6 73417bb1-74ac-4182-ab4d-53510b72aa10
Unable to find subnet with name or id '73417bb1-74ac-4182-ab4d-53510b72aa10'

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/481658

[OpenStack Infra (hudson-openstack)]

Change abandoned by Brian Haley (<email address hidden>) on branch: master
Review: https://review.openstack.org/481658

[Brian Haley (brian-haley)]

The INTERFACE name was used to be generic for subnet or port, so I guess this will stay as it is.

[Akihiro Motoki (amotoki)]

The correct format is (SUBNET|subnet=SUBNET|port=PORT). INTERFACE is used as a compromise.
One idea is to change INTERFACE to (SUBNET|subnet=SUBNET|port=PORT).