Allow to run Distribution wizard on the main wiki as superadmin

Today, when running the Distribution wizard to install an empty main wiki, the DW asks to create a user , which is then used to install the flavor on the wiki and set as owner of the main wiki.

If the superadmin user is activated in the xwiki.cfg file of the instance, DW should propose, as an option, to login with superadmin to run the Distribution Wizard instead of creating a new one. I think the option to create a new user is still interesting, even if superadmin is enabled, so it should still be there, as the main option.

This usecase is especially useful when an external authentification is to be setup on the wiki in question (for example, OpenId or a trusted authentication) and creating a user just for the install of the main wiki would be useless.