provider/manual: don't alter config in provider.Open

(Follow up to #429)

I unintentionally changed other users of the validate
function, which *shouldn't* just add defaults to the
config. For example, Open should just take the config
it's given.
Merge pull request #430 from axw/provider-manual-fix-pr429

provider/manual: don't alter config in provider.Open

(Follow up to #429)

I unintentionally changed other users of the validate
function, which *shouldn't* just add defaults to the
config. For example, Open should just take the config
it's given.