(PUP-11000) Change agent_disabled_lockfile setting type to string

There is a race condition that can cause parallel or rapid puppet agent runs
to truncate the agent_disabled_lockfile.

This changes the setting type to string to ensure that a file resource is not
added to the catalog.

Author: gcampbell12

lib/puppet/defaults.rb

@@ -1768,7 +1768,7 @@ def self.initialize_default_settings!(settings)
     },
     :agent_disabled_lockfile => {
         :default    => "$statedir/agent_disabled.lock",
-        :type       => :file,
+        :type       => :string,
         :desc       => "A lock file to indicate that puppet agent runs have been administratively
           disabled.  File contains a JSON object with state information.",
     },