rhel-systemd: start vswitchd after udev

rhel-systemd: start vswitchd after udev

It's possible to race with the udev service, such that dpdk ports are
not finished being bound until after ovs-vswitchd has been started.
This means that attempts to use the port will fail.  While it is
possible to work around this for some NICs using port hotplug, not all
port types are supported (for instance vfio), and it requires manual
intervention.

Fixes: 36af136b690c ("rhel-systemd: Delay shutting down the services")
Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=1397299
Suggested-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Tested-by: Karthik Sundaravel <ksundara@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>