* Introduce new convention for config definition.

We need to define a way to acquire configuration schema as a part of
plugin definition. Having schema we can split steps of
config validation and plugin instantiation.

* Discover plugins, read their schema and validate the config.

Config validation finished before core services and plugins read from it.
That allows us to fail fast and have predictable validation results.

* Instantiate plugins using DiscoveredPluginsDefinitions.

* Update tests for new API.

* test server is not created if config validation fails

* move plugin discovery to plugin service pre-setup stage.

Set validation schemes in ConfigService.preSetup stage.

* fix eslint problem

* generate docs

* address Rudolfs comments

* separate core services and plugins validation

* rename files for consistency

* address comments for root.js

* address comments #1

* useSchema everywhere for consistency. get rid of validateAll

* plugin system runs plugin config validation

* rename configDefinition

* move plugin schema registration in plugins plugins service

plugins system is not setup when kibana is run in optimizer mode,
so config keys aren't marked as applied.

* cleanup

* update docs

* address comments