feat(infra): update 3n-alt jobspecs with mlx cx6

The lists prioritize the new Mellanox Connect-X6 NIC but retain a small
number of XL710 test cases to preserve at least some coverage.
They also add the avf driver for the XL710 NIC to ensure the driver is
tested in at a small amount.

Signed-off-by: Maciek Konstantynowicz <mkonstan@cisco.com>
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Change-Id: Id399d075d94344fdedbe4ab71d98a33fe37f913f