Simplify add_user_to_group function

Current SLE12 and openSUSE13.X versions can handle usermod's '-a' and '-G'
switches so remove the special case.

Change-Id: If0f1390a0eb8f41ffffca74525a4648cfe8ea61d
diff --git a/functions-common b/functions-common
index ff92611..3a2f5f7 100644
--- a/functions-common
+++ b/functions-common
@@ -1842,16 +1842,7 @@
     local user=$1
     local group=$2
 
-    if [[ -z "$os_VENDOR" ]]; then
-        GetOSVersion
-    fi
-
-    # SLE11 and openSUSE 12.2 don't have the usual usermod
-    if ! is_suse || [[ "$os_VENDOR" = "openSUSE" && "$os_RELEASE" != "12.2" ]]; then
-        sudo usermod -a -G "$group" "$user"
-    else
-        sudo usermod -A "$group" "$user"
-    fi
+    sudo usermod -a -G "$group" "$user"
 }
 
 # Convert CIDR notation to a IPv4 netmask