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

doc/localhome.md

@@ -0,0 +1,37 @@
+# localhome
+
+## Description / Use cases
+Sometimes a local home directory makes more sense than the home on a network share.
+
+For example, with:
+* Laptops with Wi-Fi connectivity
+* Laptops that are also used outside the school network (e.g., teacher devices)
+* Devices assigned to a specific user who synchronizes data using a sync client
+
+## Requirements
+Nothing
+
+## Activation / Default
+To enable the local home directory, set the variable `localhome` to `true` (default: false).
+
+## Example
+
+* Hosts in group `laptops` will get the localhome.
+* All computers in the desktop group, except the PC with IP address 10.0.2.33, will have their home directory on the network (default).\
+  The PC with IP address 10.0.2.33 also has a local home directory.
+* Hosts in group `desktop` will have home on net (default)
+
+inventory.yml
+```
+laptops:
+  hosts:
+    10.0.1.[1-64]:
+  vars:
+    localhome: true
+desktops:
+  hosts:
+    10.0.2.[1-32]:
+    10.0.2.33:
+      localhome: true
+    10.0.2.[34-64]:
+```