Expand registry URL before login.

The behavior of `login` differs from `push` and `pull` as well as the command-line client in the following two aspects:
* It does not expand the registry URL, if the `registry` argument was not specified including a protocol and the path `v1`. This leads to a mismatch when caching and reusing the credentials (in `_auth_configs`).
* It accepts insecure connections by default.

Applying the same expansion as `push`/`pull` provides more consistency.