Replace "nova" entries in iptables_manager with "neutron"

In iptables_manager docstrings there are still some references to
nova left from nova/network/linux_net.py.
Remove these references and update the docstrings.

Change-Id: I50813e3050f7f117512bde7c2563ce3410939e54
Closes-Bug: #1389601
Merge "Replace "nova" entries in iptables_manager with "neutron""