Use variable instead of group membership to install localhome

Use variable localhome to determines whether the localhome module is installed.
Default: localhome=false

Further changes:
- Move pam-exec from common-auth to common-session
- Move pam-mkhomedir before pam-mount to avoid double login on first use
  on localhome devices

roles/lmn_localhome/tasks/main.yml

@@ -2,8 +2,8 @@
 - name: enable pam_mkhomedir.so
   lineinfile:
     dest: /etc/pam.d/common-session
-    line: "session  optional        pam_mkhomedir.so  umask=0077"
-    insertbefore: "session  optional        pam_mount.so"
+    line: "session optional        pam_mkhomedir.so  umask=0077"
+    insertbefore: '^session\s*optional\s*pam_mount.so'
 
 - name: Patch sddm login screen to inform about localhome
   blockinfile: