[FIX] startup: when preloading a registry with the -d option, document_webdav

[FIX] startup: when preloading a registry with the -d option, document_webdav
could not register itself because the refactoring of the startup
script run the services after the registry preloading. I would
still prefer to start the services after, but we first need the
required hooks in the addons, which would allow us to remove any
side-effecting code from them.

bzr revid: vmt@openerp.com-20110905103619-vhp1goqy7w1nmefu