stress/actions/ssh_floating.py n-net compatibility

ssh_floating.py did not worked with nova network because of the
used security group specification style.

Now it is expected to work regardless neutron or n-net used for
networking.

Change-Id: I14000e97fb6a857c7c0fed126a90e7e16509d8ec
Merge "stress/actions/ssh_floating.py n-net compatibility"