Add capability to wait for IPv6 address in ip_lib

Add capability to wait for IPv6 address in ip_lib

When an IPv6 address is added to an interface, it
goes into a tentative state for a couple of seconds
for duplicate address detection. During this time,
use of the address will fail. This is an issue for
functional tests where they may add an address to
an interface and then immediately run a ping and
expect success.

This patch adds a new wait_until_address_ready function
to ip_lib that will poll the interface every 200 ms until
the status transitions off of tentative or until a time limit
is exceeded. If the time limit is exceeded, it will raise an
exception.

It also adds unit tests and updates a functional test to
make use of the new feature.

Change-Id: I2fa51e3f55847f7b5062bec0c1c666f5c11364d5