Use PHP built-in web server instead of Apache in Drone

Instead of running an additional Drone service with the Nextcloud server
now the Nextcloud server is run in the same Drone step as the acceptance
tests themselves using the PHP built-in web server.

Thanks to this, the Nextcloud server control is no longer needed, as the
acceptance tests can now directly reset, start and stop the Nextcloud
server. Also, the "nextcloudci/php7.0:php7.0-7" image provides
everything needed to run and manage the Nextcloud server (including the
Git command used to restore the directory to a saved state), so the
custom image is no longer needed either.

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