FIX: Change plugin default behavior for Container tests

FIX: Change plugin default behavior for Container tests

Looks like disabling all plugins in container leaving
only memif_plugin.so results into issue that could lead
into lost of SSH connectivity to TB or in better case
to not handling any traffic.

This fix is supose to return previous behavior for Container
instance of VPP.
vpp_config.add_plugin('disable', 'dpdk_plugin.so')

Change-Id: I7bca6c73c0f5aaf30d423b90ff7c2d2c38d54700
Signed-off-by: Peter Mikus <pmikus@cisco.com>
(cherry picked from commit 5fe3ed050690e465c09692230e8eb715552c408f)