avocado: Add functional_tests dir, with doc build test

The shell script one liner currently executed in avocado
has some shortcomings and won't actually detect errors
building the documentation. Let's use a more proper script
to do the checking.

Since we are going to have a functional suite test for
avocado anyway, create the dir functional_tests.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Add doc build test

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