Error code for creating duplicate floating_ip_bulk

Trying to create a floating_ip_bulk that already
exists gives HttpBadRequest error, but it should
raise HttpConflict as the user is conflicting the
already created floating_ip_bulk.
So updated the code to HttpConflict error.
Closes-Bug: #1374260

Change-Id: I8194f08e6f8d1922b7316d302837549b5c957ba9
Merge "Error code for creating duplicate floating_ip_bulk"