avocado.core.output: Honor --silent unconditionally

When passing --silent to the avocado command line,
stop emiting messages to the logging system altogether.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.job: Show human output when none of the other plugins claim stdout usage

That was a wrong behavior of our application. When providing
a command line such as:

scripts/avocado run sleeptest --json /tmp/json

This command should display human output. This patch fixes this
issue, and corrects the functional test for this behavior.

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