Allow ocs/v2.php/cloud/... routes

One of the possibilities of the old OCS API is that you can define the
url yourself.

This PR makes this possible again by adding an optional root elemenet to
the route. Routes are thus:

.../ocs/v2.php/<root>/<url>

By default <root> = apps/<app>

This will allow for example the provisioning API etc to be in
../ovs/v2/php/cloud/users