Group.hs: add 'allTags'; adjust loaders and test data for it

This commit adds a Group.allTags field to store the tags of node groups,
and teaches each loader backend in HTools to populate it (additionally, the
IAllocator class in lib/cmdlib.py now includes tags for groups too). Test
data is updated to include an empty set of tags for node groups in all
affected test cases.

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