addons: dhcp: check if iface has link-local address before starting dhclient6

addons: dhcp: check if iface has link-local address before starting dhclient6

Ticket: CM-13248
Reviewed By: Roopa, Kanna, Nikhil G
Testing Done: See bug

Today before starting dhclient6, we are sleeping 2 seconds we need to make sure
the configured interface is up and has a link-local address.
In some cases 2 seconds is not enough. This patch will install a retry loop
with a 10 sec timeout.
We are querying ip -6 addr show to make sure the interface is properly confi-
-gured but in the future the plan is to move this call to netlink.

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>