Currently, the only way to pass parameters to Avocado is by using the
variants layer, that is, one of the existing varianter plugin
implementations.  This was based on earlier design decisions, that
we're trying to fix without breaking expectation and compatibility.

While it'd makes sense to support parameters created by the varianter
layer itself, together with non-varianter parameters, it seems that we
can start by adding support for one or the other being used at a time.

This commit does exactly that: add support for a method of passing
parameters via the command line for all tests in a job, without
having to resort to the varianter.  When the varianter is used,
it takes precedence and the core test parameter passing support
is ignored.

This should solve a number of use cases that require one-time
executions of tests with different parameters, especially
executions during test development and debugging.
Signed-off-by: NCleber Rosa <crosa@redhat.com>