Initialize the path.settings inside the runner class

The `path.settings` requires the LOGSTASH_HOME constant to be defined,
the problem is that constant is only defined when you are actually
inside the logstash application, This was causing a bug when you were
testing plugin individually because that constant wasn't defined.

Fixes: #5361

Fixes #5363

logstash-core/lib/logstash/environment.rb

@@ -13,7 +13,6 @@ module LogStash
            Setting::Boolean.new("config.reload.automatic", false),
            Setting::Numeric.new("config.reload.interval", 3),
            Setting::Boolean.new("metric.collect", true) {|v| v == true }, # metric collection cannot be disabled
-            Setting::String.new("path.settings", ::File.join(Environment::LOGSTASH_HOME, "config")),
             Setting::String.new("pipeline.id", "main"),
            Setting::Numeric.new("pipeline.workers", LogStash::Config::CpuCoreStrategy.maximum),
            Setting::Numeric.new("pipeline.output.workers", 1),

logstash-core/lib/logstash/runner.rb

@@ -15,8 +15,14 @@ require "logstash/agent"
 require "logstash/config/defaults"
 require "logstash/shutdown_watcher"
 require "logstash/patches/clamp"
+require "logstash/settings"
 
 class LogStash::Runner < Clamp::StrictCommand
+  # The `path.settings` need to be defined in the runner instead of the `logstash-core/lib/logstash/environment.r`
+  # because the `Environment::LOGSTASH_HOME` doesn't exist in the context of the `logstash-core` gem.
+  # 
+  # See issues https://github.com/elastic/logstash/issues/5361
+  LogStash::SETTINGS.register(LogStash::Setting::String.new("path.settings", ::File.join(LogStash::Environment::LOGSTASH_HOME, "config")))
 
   # Node Settings
   option ["-n", "--node.name"], "NAME",
@@ -129,7 +135,9 @@ class LogStash::Runner < Clamp::StrictCommand
 
   def run(args)
     settings_path = fetch_settings_path(args)
+
     @settings.set("path.settings", settings_path) if settings_path
+
     LogStash::SETTINGS.from_yaml(LogStash::SETTINGS.get("path.settings"))
     super(*[args])
   end