memoize root_user/root_group methods

Summary:
We don't expect the platform to change mid-run, so memoizing these
methods to save calls to `#value_for_platform`, which has node attribute reads
under the hood.

Differential Revision: D55338996

fbshipit-source-id: 7c563a1f5f13c58b35b081b2796143b9d6939cd7

cookbooks/fb_helpers/libraries/node_methods.rb

@@ -990,7 +990,7 @@ def solo?
     end
 
     def root_user
-      value_for_platform(
+      @root_user ||= value_for_platform(
         'windows' => { 'default' => 'Administrator' },
         'default' => 'root',
       )
@@ -999,7 +999,7 @@ def root_user
     def root_group
       # rubocop:disable Chef/Correctness/InvalidPlatformValueForPlatformHelper
       # See the `macos?` method above
-      value_for_platform(
+      @root_group ||= value_for_platform(
         %w{openbsd freebsd mac_os_x macos} => { 'default' => 'wheel' },
         'windows' => { 'default' => 'Administrators' },
         'default' => 'root',