Support for disabling anti spoofing

Modified the iptables_firewall file to add support for disabling anti spoofing in /etc/neutron/neutron.conf. The parameters read in the latter are "disable_anti_spoofing = True" (or False) and "security_group_name = nameDefinedByUser". Those 2 parameters are optional for this to work (if they don't exist, default behavior will take place).
At this point, the user can create a security group with the name defined in neutron.conf, and after creating a port that uses this security group, any instances spun up and associated with that port will have anti spoofing disabled.

Nabil M.
nmaadara@cisco.com