From: Tomáš Ďurovec <tomas.durovec@pantheon.tech>

Given the whole DPDK tree directory can now be copied to the nodes,
there is no more need to use the git ref option, as the tree can be
controlled directly by the user.

Signed-off-by: Tomáš Ďurovec <tomas.durovec@pantheon.tech>
Signed-off-by: Luca Vizzarro <luca.vizzarro@arm.com>