This removes the exception of allowing failures on the Travis CI jobs
with Python 3, while, at the same time, temporary restricts Python 3
jobs to run only unittests and the documentation build for the Avocado
core.  That is, it excludes the functional tests, and the optional
plugin's tests.

While it may look like a step back, it's actually a step forward in the
Python 3 port, because now any regression on the unittests will be caught.
And, when the additional fixes are applied to address the functional tests
and plugin's tests, the whole set of tests will be enabled for Python 3.
Signed-off-by: NCleber Rosa <crosa@redhat.com>