Downgrade and fix log message for floating ip already disassociated

When a floating IP is associated, try to disassociate may
get an FloatingIpNotAssociated exception. This patch
fixes the log message and downgrades the log
level as it is something that will reasonably occur.

Change-Id: I1d4d5fe916d99024ff4df53d300a0f26d74eb102
Merge "Downgrade and fix log message for floating ip already disassociated"