Catching new exception while disassociating a disassociated floating ip

Because the response of disassociating floating ip api will be
changed(bug #1129719), so the tempest codes need to be updated synchronically.

Reverted the commit 49e330611319f4096061a92b4fad61a9c482b76f, and add a new
exception UnprocessableEntity.

Change-Id: I2b5944599a86f5805375f0d2d49e4ab38dc05825
Merge "Catching new exception while disassociating a disassociated floating ip"