[logstash][x-pack] configuration inconsistency with elastic & kibana

In Elastic and Kibana, when x-pack is installed but the security is disabled, you have the following setting in config file:
```
xpack.security.enabled: false
```

But in logstash, this setting does not exist.
Instead, you have to provide and empty login and an empty password :
```
xpack.monitoring.elasticsearch.url: http://localhost:9201
xpack.monitoring.elasticsearch.username:
xpack.monitoring.elasticsearch.password:
```

Why not unify settings ?

Version : 6.0.0rc1

Start by reviewing the Logstash monitoring configuration documented in the issue, especially xpack.monitoring.elasticsearch.url, username, and password, and compare it with the xpack.security.enabled behavior described for Elastic and Kibana. Determine whether a unified disabled-security setting is feasible and document the expected configuration and validation for version 6.0.0rc1.

Written by the indexing model from the issue text.