Add Conflict exception in test_floating_ips_actions case

We modify the error return code in nova api both v2 and v2.1, so we need to
reflect this to tempest test cases test_associate_already_associated_floating_ip
changes : add HTTPConflict

Nove review link: https://review.openstack.org/#/c/127889/

Change-Id: Icbb7b0aa46c864993290749e9a6711f3fa224255
Merge "Add Conflict exception in test_floating_ips_actions case"