Display only reachable IP as Floating IP association target

In Neutron deployments some VM port can be unreachable from
external network and cannot be associated with floating IP.
It is confusing if these ports are listed in Floating IP
Associate form.

Change-Id: I2d8faf0dbf4490d198b883fe1becfd950b1b4d14
Closes-Bug: #1252403
Merge "Display only reachable IP as Floating IP association target"