[fdio] Pin VPP packages to 18.07-release

[fdio] Pin VPP packages to 18.07-release

VPP 18.10 has a weird bug triggered by certain packets, e.g. from
inside a guest VM on a compute node, these behave differently:
$ udhcpc -x hostname:1234567890123456789012  # works
$ udhcpc -x hostname:12345678901234567890123 # confuses VPP on gtw01

To avoid this bug, pin VPP to the previous release, which does not
exhibit the issue.

Change-Id: I8c1e085731909d4b9296e8b09608887a4b5bfdd6
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 8ea1edb53676fdf074694967368747bd6919d682)