Yes in CentOS8 network-script is not installed/enabled by default. So to manage ovs bridges you either need to install/enable network-scripts or manage bridges with NetworkManager via nmcli or other tools.
I have pushed some fixes to doc[1] and packstack[2] to have bridges work with network-scripts and remove Network Manager warning.
# Install and Enable ovs plugin for NetworkManager
sudo dnf install -y NetworkManager-ovs
sudo systemctl restart NetworkManager
# Discover network configs for external interface
export IP=`ip r get 1.1.1.1 | awk '/dev/{print $7}' | tr -d '[[:space:]]'`
export NETMASK=`ip route show | grep $IP |cut -d" " -f1|cut -d"/" -f2 |tr -d '[[:space:]]'`
# We need the gateway as we'll be reconfiguring the interface
export GATEWAY=`ip r get 1.1.1.1 | awk '/dev/{print $3}' | tr -d '[[:space:]]'`
export MAC=`ip a show enp0s3|grep link/ether|awk '{print $2}' | tr -d '[[:space:]]'`
# Delete ovs bridge created outside of NetworkManager
sudo ovs-vsctl del-br br-ex
sudo nmcli conn add type ovs-bridge conn.interface br-ex
sudo nmcli conn add type ovs-port conn.interface port0 master br-ex
sudo nmcli conn add type ovs-interface slave-type ovs-port conn.interface br-ex master port0 ipv4.method manual ipv4.address $IP/$NETMASK ipv4.gateway $GATEWAY ipv4.dns 8.8.8.8
# This do no't persist after reboot
sudo ovs-vsctl set bridge br-ex other-config:hwaddr=$MAC
sudo nmcli conn add type ovs-port conn.interface port1 master br-ex
sudo nmcli conn add type ethernet conn.interface enp0s3 master port1
sudo nmcli conn up ovs-slave-enp0s3
# to persist mac on ovs bridge with reboots
sudo tee /etc/NetworkManager/dispatcher.d/25-ovs-br-ex <<EOF
#!/bin/sh
# This is a NetworkManager dispatcher / networkd-dispatcher script for
# ovs to set mac address for slave
# For NetworkManager consider only up/down events
[ 0 -ge 2 ] && [ "" != "up" ] && exit 0
if [ "\$DEVICE_IFACE" == "br-ex" ]; then
export MAC=\$(ip a show enp0s3|grep link/ether|awk '{print \$2}' | tr -d '[[:space:]]')
ovs-vsctl set bridge br-ex other-config:hwaddr=\$MAC
fi
exit 0
EOF
sudo chmod +x /etc/NetworkManager/dispatcher.d/25-ovs-br-ex
Ensure you have console access to the packstack vm as in case of wrong network config ssh connection will be lost.