avocado.job: Process multiplex files

If one passes a multiplex file in the runner arguments,
we shall use it and generate test dicts for the entire
test matrix, and pass those dicts to each test instance.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.runner: Introduce --multiplex-file option

With this, people will be able to introduce a --multiplex-file
(short -m) option to the run subcommand, that provides the
path to a multiplex file, that will be turned into a test
matrix (list of dicts) that will be run by the test runner.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
sleeptest: Modify to make use of self.params/multiplex file

Modify sleeptest to pick any params from self.params.
Also, introduce a sample multiplex file, sleeptest.mplx,
an example of how to write a multiplex file to fulfill
matrix defining goals.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Add functional tests for multiplex

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>