Add antispoofing option for CSR

Add support for a 'disable_anti_spoofing' option in the [DEFAULT] block
of /etc/neutron/neutron.conf. If set, a second option
'sec_group_svc_VM_name' will be enabled which lists a security group
that does not have Neutron anti-spoofing rules applied. Together the
two options allow disabling of anti-spoofing rules on a per-port basis
as needed for service VMs.

Code contributed by Ly Loi and Nabil Maadarani.

Closes-Bug: #1343401