NPE in PluginsService when starting elasticsearch with a wrong user (Fix for 4186)

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)

Related discussion: https://groups.google.com/forum/#!topic/elasticsearch/_WRW4Qfpo7M

Closes #4186.