Use `/usr/share/pam-configs` for changes to `/etc/pam.d`

[dorianim]

We should use /usr/share/pam-configs to modify the /etc/pam.d/common-session file. This makes it update-proof.
See here:
https://help.ubuntu.com/community/LDAPClientAuthentication#Automatically_create_home_folders
and here:
https://wiki.ubuntu.com/PAMConfigFrameworkSpec