Change error code of floating_ip_dns api(v2.1)

Changes :
webob.exc.HTTPUnprocessableEntity(422) -> webob.exc.HTTPBadRequest(400)

Partially implements blueprint v2-on-v3-api
Change-Id: Ic6063c2c4525c7ff8e07495c75c134ff7e78756b
Merge "Change error code of floating_ip_dns api(v2.1)"