Warn operators not to install openvswitch-brcompat

The LibvirtHybridOVSBridgeDriver VIF driver for the Open vSwitch
Neutron plugin needs to create a Linux bridge using brctl for the
iptables firewall rules that implement security groups. But if the
package openvswitch-brcompat is installed, brctl actually creates an
OVS bridge and the security groups won't work.

bug: #1220358

Cherry-picked from https://review.openstack.org/45061

Change-Id: Ia74ffbb4ed327af41f938f900fc105492606b50b