Disassociate floating ips on port deletion

* When doing a port delete, neutron does not clean up floating ips associated
  with the port. This means that the plugin code has to handle this explicitly

Signed-off-by: Joe Mills <joe@midokura.com>

Fixes MN-1072
Merge pull request #67 from JoeMido/MN-1072

Disassociate floating ips on port deletion