Improve error return code of floating_ips in v2/v2.1 api

1.change exception.InvalidID from 404 to 400 when passing a invalid
id.

2.handle exceptions when calling get_instance_id_by_floating_address()
  FloatingIpNotFoundForAddress -> HTTPNotFound
  FloatingIpMultipleFoundForAddress ->HTTPConflict

3.change HTTPUnprocessableEntity to HTTPConflict when remove a floating
ip which is not associated the instance

Change-Id: I0c9cd5b50a2d3cf024e5c4c3506ae2d0e6394c88
Merge "Improve error return code of floating_ips in v2/v2.1 api"