Make possible to configure the domain for the Nextcloud test server

The NextcloudTestServerLocalHelper started the PHP built-in web server
for the Nextcloud test server at 127.0.0.1; as the Selenium server has
to access the Nextcloud test server they were forced to share the same
network. Now, the domain at which the PHP built-in web server is started
can be specified when the NextcloudTestServerLocalHelper is created,
which removes the need of sharing the same network, as the Selenium
server now can access the Nextcloud test server at an arbitrary domain.
However, by default "127.0.0.1" is still used if no domain is given.

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