core: Add support for plugins to hook into the environment load process before configurations are parsed

lib/vagrant/environment.rb

@@ -131,6 +131,10 @@ module Vagrant
       # Load the plugins
       load_plugins
 
+      # Call the hooks that does not require configurations to be loaded
+      # by using a "clean" action runner
+      hook(:environment_plugins_loaded, Action::Runner.new)
+
       # Call the environment load hooks
       hook(:environment_load)
     end
@@ -268,10 +272,10 @@ module Vagrant
     # against the given name will be run in the context of this environment.
     #
     # @param [Symbol] name Name of the hook.
-    def hook(name)
+    def hook(name, runner = action_runner)
       @logger.info("Running hook: #{name}")
       callable = Action::Builder.new
-      action_runner.run(
+      runner.run(
         callable,
         :action_name => name,
         :env => self)