web UI is not accessible due to lack of permissions

[Patricia Domingues]

rabbitmq-server charm has the config option to enable the management plugin (management_plugin), but when you set it to true the user doesn't have the proper Rabbitmq-server tag as required (https://www.rabbitmq.com/management.html#permissions) so, you are not able to access the web interface.

[Patricia Domingues (patriciasd)]

after changing rabbitmq-server config to enable the plugin:

```
juju config rabbitmq-server management_plugin=true
```

After that, I see that the port (15672) is opened. In `juju status`, I can see `5672/tcp,15672/tcp`

When I try to access Web UI it says `Login failed`, and rabbitmq-server log shows:

```
=WARNING REPORT==== 15-Mar-2019::20:31:32 ===
HTTP access denied: user 'testui' - Not management user
```

[Patricia Domingues (patriciasd)]

Also, `sudo rabbitmqctl list_users` shows:

Listing users
guest [administrator]
testui []