addons: dhcp: stop IPv6 dhclient on ifdown and add delay before starting dhclient

addons: dhcp: stop IPv6 dhclient on ifdown and add delay before starting dhclient

Ticket: CM-11043
Reviewed By: roopa
Testing Done: used the config mentioned in bug

This patch adds a delay before starting IPv6 dhclient to
make sure the configured interface/link in up.

Also, ifdown on a IPv6 interface, this patch makes sure
that the link in up before dhcp release. Dhcp release is
done using a default lease file.

Signed-off-by: Nikhil <nikhil@cumulusnetworks.com>