Do not release FIP on disassociate action

In Instances panel, associate and disassociate
Floating IPs now will be consistent.

Fixes bug #1247245

Change-Id: I6a84c43b8bc69fa08c63abff5343144cc4000c37
(cherry picked from commit 348ffe41c49a9a23c9c7da65cbceaf363b13a95e)
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules