Allow to configure password_autocomplete = on/off

Horizon does not allow username/password auto-completion with browsers by default by putting a fake form before a real login form. It would be arguable to allow password completion by browsers, but it helps user to configure more complicated/strong password. It would be nice if the charm supports to configure on/off of password auto-completion.

[openstack_dashboard/local/local_settings.py.example]
# Turn off browser autocompletion for forms including the login form and
# the database creation workflow if so desired.
#HORIZON_CONFIG["password_autocomplete"] = "off"

[horizon/templates/auth/_login_form.html]
      {% if HORIZON_CONFIG.password_autocomplete != "on" %}
        <div class="fake_credentials" style="display: none">
          <input type="text" name="fake_email" value="" />
          <input type="password" name="fake_password" value="" />
        </div>
      {%endif%}