use floatingip's ID as key instead of itself

Change-Id: I267f174a0e2e611ec007404c7b44e2c73a359c38
Closes-Bug: 1294526
(cherry picked from commit 7120325d37df414d8de084c5b41fdfbea3a06565)
Merge "use floatingip's ID as key instead of itself" into milestone-proposed