config.py: replace "node group" with "nodegroup"
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 <16293852a0900193fe70fa7d24f714f37ba99a56@google.com>
Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

leshchevds/ganeti,bitemyapp/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,leshchevds/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,grnet/snf-ganeti,andir/ganeti,ganeti/ganeti,bitemyapp/ganeti,yiannist/ganeti,andir/ganeti,onponomarev/ganeti,mbakke/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,apyrgio/snf-ganeti,dimara/ganeti,onponomarev/ganeti,apyrgio/ganeti,dimara/ganeti,ganeti/ganeti,yiannist/ganeti