Add runner management health check for /ping

Even though we can check the health of the runner management, we would like to ensure an external health check validates the availability of all required services.

lib/runner/strategy/null.rb

@@ -45,6 +45,10 @@ class Runner::Strategy::Null < Runner::Strategy
 
   def self.config; end
 
+  def self.health
+    true
+  end
+
   def self.release
     'N/A'
   end