abrt: Improve retrace user modification flow

We need to stop and restart the httpd service when modifying the retrace
user, otherwise we get errors like "user retrace is currently used by
process <PID>".

Also, factor out some magic values into role variables.

roles/abrt/retrace-pre/defaults/main.yml

@@ -0,0 +1,3 @@
+---
+retrace_user_home: /srv/retrace/home
+retrace_user_uid_gid: 174

roles/abrt/retrace-pre/tasks/main.yml

@@ -1,12 +1,42 @@
 ---
-# create uid - this is normaly done by retrace package
-# but the package is not installed yet at this momement
-- name: Create retrace user
+- name: Make sure retrace user exists
   user:
     name: retrace
     group: retrace
-    uid: 174
-    home: /srv/retrace/home
+    system: yes
+    create_home: no
+  register: retrace_user_info
+
+- name: Ensure retrace user has correct UID and home directory
+  when: >
+    retrace_user_info.changed or
+    retrace_user_info.uid != retrace_user_uid_gid or
+    retrace_user_info.home != retrace_user_home
+  block:
+  - name: Stop httpd service before modifying user
+    service:
+      name: httpd
+      state: stopped
+
+  # create uid - this is normaly done by retrace package
+  # but the package is not installed yet at this momement
+  - name: Ensure correct UID and home directory
+    user:
+      name: retrace
+      uid: "{{ retrace_user_uid_gid }}"
+      gid: "{{ retrace_user_uid_gid }}"
+      home: "{{ retrace_user_home }}"
+      create_home: yes
+
+  - name: Ensure correct GID
+    group:
+      name: retrace
+      gid: "{{ retrace_user_uid_gid }}"
+
+  - name: Restart httpd service after modifications are done
+    service:
+      name: httpd
+      state: started
 
 - name: Create directory for repositories
   file: