tests: allow for externally supplied VPP workers config for tests

tests: allow for externally supplied VPP workers config for tests

Allow to supply the external VPP worker config for tests which
do not specify the workers config explicitly, and use
the tags infra to flag those that need attention in this configuration.
This commit shows one example use of such a tag, there will be
a separate commit with the rest of the places needing it,
since that change is rather mechanical.

Thus, the assumption is that the test should by default be agnostic
of the VPP configuration, unless it explicitly specifies so.

Type: test
Change-Id: I3c0077e4e22a75cb9561fb98d3b783b93486b2be
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>