config.py: replace "node group" with "nodegroup"

Though code uses "nodegroup" as data member name, strings that will be
read by the user should use "node group" (with a space), since "nodegroup"
is not a word.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>