Add exceptions.BadRequest to the test case of reserve_fixed_ip

we change the exception of reserve_fixed_ip in fixed_ip api to
HTTPBadRequest, add this exception to tempest, and also we need
to accept HTTPNotFound exception to support juno test cases.

Change-Id: Iafd2fb82ad5d50c2f4b3f0819ac7a6f8aa901264
Merge "Add exceptions.BadRequest to the test case of reserve_fixed_ip"