Run functional tests under tox

This patch allows to run functional tests under tox, however, they are
disabled by default. In order to run tests with tox, it is necessary to
enable them by modifying the functional-tests.conf and setting run_tests
to True.

This patch also merged both test files in a single one
(test-requirements.txt)

blueprint refactor-system-tests

Change-Id: Iba440b12653cf9ee1454022830107a06b8a7bea9