[WIP] 1111591 - require passwords to not be empty strings

This is a work around to fix [BZ-1111591](https://bugzilla.redhat.com/show_bug.cgi?id=111159). A better solution would be add an optional validator to Okaara's `prompt_password`.

Currently empty passwords cannot work because the password will be assigned the next value it is given. The command:

    pulp-admin -u user -p  login

takes the argument "login" as a password.

We should not allow the user to get into an unusable state.