Remove _security_group_chain_name from nova/virt/firewall.py

The function _security_group_chain_name in nova/virt/firewall.py is
useless for now, just remove it.

Change-Id: I1ad7050115ad56ce7b152efbcfcc42aff8e6c3ed
Merge "Remove _security_group_chain_name from nova/virt/firewall.py"