Disallow self-reset and preserve old facade version.

[anastasiamac]

Description of change

Users are no longer allowed to reset their own password. If a user wants to have anew password, they can use normal "change password" workflow. Unit test are adjusted accordingly.

This PR also ensures that there exist a previous facade version for backward compatibility.

QA steps

1. bootstrap
2. run 'juju change-user-password --reset' or 'juju change-user-password --reset '
3. Either command in 2 should be disallowed.

Documentation changes

n/a

Bug reference

n/a

[anastasiamac]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju