owncloud httpd-service: add urlPrefix option

This option allows user to specify a url prefix for owncloud.
By default it is set to "" and the document root will be set
to owncloud's dir.
If a prefix is set, e.g. urlPrefix = "/owncloud"
an alias will be created using that prefix to point to owncloud's
dir and owncloud will be available at http://localhost/owncloud