zebra: restart start-up delay timer when the first uplink comes up

zebra: restart start-up delay timer when the first uplink comes up

When all the uplinks go down the VTEP is disconnected from the
VxLAN overlay and this was handled by proto-downing the ES bonds. When
the uplinks come up again we need to re-enable the ES bonds but that
needs to be done after a delay to allow the EVPN network to converge.

And that is done by firing off the startup-delay timer on first
uplink-up.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>