Add missing migrated fields for system module. (elastic#11545)

* This adds all missing migrated fields for the system module to ecs-migration.yml
* The breaking changes doc has been updated accordingly with script/renamed_fields.py
* The corresponding dashboards for the system module have been updated manually (sorry for all the whitespace diff)

dev-tools/ecs-migration.yml

@@ -463,7 +463,26 @@
 - from: system.auth.hostname
   to: host.hostname
   alias: true
-  copy_to: false
+  beat: filebeat
+
+- from: system.auth.message
+  to: message
+  alias: true
+  beat: filebeat
+
+- from: system.auth.program
+  to: process.name
+  alias: true
+  beat: filebeat
+
+- from: system.auth.timestamp
+  to: '@timestamp'
+  alias: true
+  beat: filebeat
+
+- from: system.auth.user
+  to: user.name
+  alias: true
   beat: filebeat
 
 - from: system.auth.pid
@@ -476,12 +495,22 @@
   alias: true
   beat: filebeat
 
+- from: system.auth.groupadd.name
+  to: group.name
+  alias: true
+  beat: filebeat
+
+- from: system.auth.useradd.gid
+  to: group.id
+  alias: true
+  beat: filebeat
+
 - from: system.auth.useradd.uid
   to: user.id
   alias: true
   beat: filebeat
 
-- from: system.auth.useradd.user
+- from: system.auth.useradd.name
   to: user.name
   alias: true
   beat: filebeat