build: add ability to disable some plugins from packaging and tests

When custom-packaging the VPP artifacts, it can be useful to exclude
some of the core plugins from packaging/testing, for some reasons.
A removal of a plugin(s) from the worktree needs to be tracked as
a separate change, and thus is tricky from the maintenance
point of view.

This change adds the ability to "pretend they do not exist" -
plugins which are added to the comma-separated environment
variable "VPP_EXCLUDED_PLUGINS" will not be added to the build
process and not packaged.

The tests do not have the 1:1 relationship as plugins,
so they might need to be modified separately. This change
includes some of these modifications as an example.

Type: feature
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: Id31562d00a01ced1acbb4996a633517cbd6f09d8