Fix crash when deploying with implicit os name

When booted into an ostree you can deploy without passing
an --os option. That was crashing though, because
ot_admin_complete_deploy_one is called with NULL
osname but it was not handling it properly.

https://bugzilla.gnome.org/show_bug.cgi?id=710970
main: Treat default osname more consistently

The libostree already treats passing NULL for osname as "booted
osname, if any".  We should do the same inside the tools.  The upgrade
builtin had this logic duplicated there; we should be able to safely
remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=710970