Return 404 instead of 501 for unsupported actions

Change unsupported actions return code to 404 from 501 to ensure
consistency with other unsupported actions in the REST API.

Change-Id: Ibf40ef9108f209b70ffbab6e2387d2d469f7373e
Backport v3 api unittest into v2 api for attach_interface extension

On most APIs, the unit test coverage of v3 is better than the one of v2.
This patch ports v3 attach_interface API tests to v2 tests for improving
v2 attach_interface API tests

Change-Id: Ie93175602dbced361d76abbe35a7767a96d9e68e
Use common get_instance function in v2 attach_interface

This patch use common get_instance function in v2 attach_interface.
That can make the instance behavior consistent with v2.1 api. Then
v2.1 and v2 can share same unittest.

Change-Id: Id531377d5d34d76a4ffafaa02ecab585cbcb369b
Merge "Backport v3 api unittest into v2 api for attach_interface extension"
Merge "Use common get_instance function in v2 attach_interface"
Merge "Return 404 instead of 501 for unsupported actions"