scenario/network_basic_ops: reassociate floating-ip

check that a detached FIP can be associated with a new VM and make sure
it is not associated with the old VM by assigning a different keypair to each
new VM so that ssh connection cannot be made without the right keypair

Change-Id: Ib535aa66638a3e7f2f1549b6e7a21a160377df6a
Merge "scenario/network_basic_ops: reassociate floating-ip"