[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. 
Rather than continue prompting, why not just print the error and exit? 'Tis a little simpler.

That said, if you prefer this behavior it's cool with me. It's a matter of preference. However, you could make it less scary than having an ominous ```while True``` in the code (that scares me every time I see it!) by setting password to the empty string before the loop and changing the while condition to ```while not password```. Then you can lose the break. It's one less line of code and raises less red flags. So something like this:

```
password = ''
while not password:
    password = self.context.prompt.prompt_password(_(prompt_msg), _(verify_msg),
                                                                                    _(unmatch_msg))
    ...
    if not password:
        <error messages>
```