Fix HTTP methods for test_attach_interfaces

delete() method of attach_interfaces API is routed when specifying
"DELETE" as HTTP method, but some tests specify "POST". This bug
does not affect the test behaviors because the tests do not check
API router behaviors and just checks delete() method behaviors.

However, this patch fixes this bug because the bug confuses the
API router behaviors.

Change-Id: I8e862cbd58da820eb336d3ef41185e16f63bb6be
Merge "Fix HTTP methods for test_attach_interfaces"