5.1.1 Known Issue about CentOS/neutron netfilter/Security Groups

/etc/sysctl netfilter parameters are set to 0 rather than 1
on CentOS.  It's too late to apply a fix to 5.1.1 so adding
to the Known Issues list.

Change-Id: I4f4e11e207bb1ce6e66e6bc526a8f62fbafa4563
Partial-Bug: 1400787