Add NextcloudTestServerHelper for Nextcloud servers in Drone services

Due to security concerns, the public Nextcloud server repository is not
set as "trusted" in Drone (otherwise a malicious pull request could be
used to take over the server), so it is not possible to create Docker
containers from the containers started by Drone. Therefore, the
Nextcloud server must be started as a service by Drone itself.

The NextcloudTestServerDroneHelper is added to manage from the
acceptance tests a Nextcloud test server running in a Drone service; to
be able to control the remote Nextcloud server the Drone service must
provide the Nextcloud server control server.

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