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/defaults/main.yml

@@ -0,0 +1,2 @@
+---
+localhome: false