Make possible to set the acceptance tests directory to use

When the acceptance tests were run they were always loaded from the
"tests/acceptance" directory of the Nextcloud server. Now it is possible
to set the directory used to look for the Behat configuration and the
Nextcloud installation script, which makes possible to run acceptance
tests for the apps too instead of only for the server (although if no
directory is explicitly given the tests for the server are the ones
run).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>