fixes #6873 - ensure hidden 'owner' is populated on user role

Also re-order seeding such that the anonymous role is added to seeded users.

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I've filed http://projects.theforeman.org/issues/6878 to consider moving away from default scopes.  Here the default (hidden) scope on User is causing a problem with the UserRole association, as owner_id is populated, but owner is nil.

If you do the 1.4 to 1.6 upgrade (see #1639 description) without the user.rb change but with the seed reorder, you'll see the error clearly, as the anonymous role will be present and get added to the hidden user.
Trying to understand the fix, why the `self.roles << role` did not the thing?