[DOCS] Fix wrong explanation in configuration.asciidoc

Replaced network.host with node.name to match config file
[DOCS] Added note about dely in clearing filter cache.

Closes #5231
NPE in PluginsService when starting elasticsearch with a wrong user

When starting elasticsearch with a wrong linux user, it could generate a `NullPointerException` when `PluginsService` tries to list available plugins in `./plugins` dir.

To reproduce:

* create a plugins directory with `rwx` rights for root user only
* launch elasticsearch from another account (elasticsearch for example)

It was supposed to be fixed with #4186, but sadly it's not :-(

Closes #5195.