Use floating_network_name instead of 'public'

In RemoteDeeplyNestedStackTest, network name is assigned as 'public'.
However, there is no guarantee to have 'public' network.
Instead of using the hardcoded network name, we should use
floating_network_name assigned in conf file.

Change-Id: I01313d557e415bb0b0d8df310c445e63190f4505
Story: #2006144
Task: 35640
1 file changed