frr: enable and start frr on reloading the controller config

frr: enable and start frr on reloading the controller config

Since we now ship frr with Proxmox VE, the frr service is available on
the nodes but disabled on install. Prior to that, users had to
manually install frr, which automatically enabled the service. When
first applying a SDN configuration with an EVPN controller, we always
fell back to restarting the frr service, because reloading fails when
the daemon isn't running. This fallback to restarting leads to the
service running but still being in the disabled state. This means that
the EVPN setup is working until the next reboot. To avoid the
situation where users configure an EVPN controller and everything
seems to be working, until a restart breaks the EVPN setup,
additionally enable and start the frr service before trying to reload
the configuration.

We enable the service after checking for the existence of
frr-pythontools in order to avoid the situation where users apply an
SDN configuration with an EVPN controller, but reloading fails due to
a missing frr-pythontools package. Since we do an early return there,
we never fell back to restarting the service in case frr-pythontools
was not available. If we enabled the service before the check, the
configuration would apply after a reboot since it already got written
to the frr configuration file.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>