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
Update documentation for clarity

Fixes bug #1393908

Change-Id: Iec46949f9f4beb25d6818b6b141de78180630a60
Merge "Add Conflict exception in test_floating_ips_actions case"
Merge "Update documentation for clarity"