'CheckInitialized' in the credential-helper library actually invokes
`pass`, which isn't desirable (see #699).
This moves the check to be simpler, and then pass will only be invoked
when it's needed (such as for `docker login` or when pulling from a
private registry).
This logic could also reasonably live in the credential-helper library,
but it's simple enough it seems fine in either location.
Signed-off-by: Euan Kemp <euank@euank.com>
056015c3d8