Add "sss" to the automount database.  The sss service ordering is
based on passwd (and others; sss comes after files) and netgroup (sss
comes after nisplus).
 
Index: b/releng/nsswitch.conf
===================================================================
--- a/releng/nsswitch.conf
+++ b/releng/nsswitch.conf
@@ -59,6 +59,6 @@ netgroup:   nisplus sss
 
 publickey:  nisplus
 
-automount:  files nisplus
+automount:  files nisplus sss
 aliases:    files nisplus